Make dependency files deterministic

The dependency file generated by Clang can sometimes contain absolute
paths to system headers, system modules and files generated by Bazel
during the build.  These paths can defer between machines and between
Bazel instances, for example:

```
bazel-out/apl-darwin_x86_64-fastbuild/bin/_objs/B/arc/B.o: B.m \
  /private/var/tmp/_bazel_user/b4422045cf7215be2d3fa4e439c7376a/sandbox/darwin-sandbox/214/execroot/__main__/bazel-out/apl-darwin_x86_64-fastbuild/bin/A.modulemap \
  /Applications/Xcode-11.6.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/System/Library/Frameworks/Foundation.framework/Modules/module.modulemap
```

This patch adds a post-process to wrapped_clang to normalize those paths
by:

* converting absolute paths to relative paths for paths to files under
the execution root
* standardizing the paths to system headers, system modules to always
use the standard location of Xcode (`/Applications/Xcode.app`).  The
user doesn't need to have Xcode installed to this location as Bazel are
already mapping them to the correct location locally.

This makes the produced `.d` files deterministic and can reliably be
shared with the remote cache.

Fixes bazelbuild#11817.

tools/osx/crosstool/cc_toolchain_config.bzl

@@ -5572,7 +5572,11 @@ def _impl(ctx):
                 ],
                 flag_groups = [
                     flag_group(
-                        flags = ["-MD", "-MF", "%{dependency_file}"],
+                        flags = [
+                            "-MD",
+                            "-MF",
+                            "DEPENDENCY_FILE=%{dependency_file}",
+                        ],
                         expand_if_available = "dependency_file",
                     ),
                 ],

tools/osx/crosstool/wrapped_clang.cc

@@ -24,6 +24,13 @@
 //    link action generating the dsym file.
 // "DSYM_HINT_DSYM_PATH": Workspace-relative path to dSYM dwarf file.
 //
+// Additionally, this post-processes the output dependency file (`.d` file) to
+// always use the relative paths to the execution root, and to standardize the
+// system header paths to always start with
+// `/Applications/Xcode.app/Contents/Developer`).  This makes dependency files
+// deterministic and cacheable, as the user may have Xcode installed to a
+// non-standard location (e.g. `/Applications/Xcode-11.6.0.app`).
+//
 // Likewise, this wrapper also contains a workaround for a bug in ld that causes
 // flaky builds when using Bitcode symbol maps. ld allows the
 // -bitcode_symbol_map to be either a directory (into which the file will be
@@ -59,6 +66,7 @@ extern char **environ;
 namespace {
 
 constexpr char kAddASTPathPrefix[] = "-Wl,-add_ast_path,";
+constexpr char kStandardDeveloperDir[] = "/Applications/Xcode.app/Contents/Developer";
 
 // Returns the base name of the given filepath. For example, given
 // /foo/bar/baz.txt, returns 'baz.txt'.
@@ -169,6 +177,25 @@ bool StripPrefixStringIfPresent(std::string *str, const std::string &prefix) {
   return false;
 }
 
+// Post-processes the dependency file to convert the absolute paths to relative paths
+// and normalize the system header paths to always start with the standard developer
+// directory.
+void PostProcessDepFile(const std::string cwd_str, const std::string developer_dir,
+                        const std::string preprocessed_depfile,
+                        const std::string depfile) {
+  if (preprocessed_depfile.empty()) {
+    return;
+  }
+  std::ifstream in(preprocessed_depfile);
+  std::ofstream out(depfile);
+  std::string line;
+  while (getline(in, line)) {
+    FindAndReplace(developer_dir, kStandardDeveloperDir, &line);
+    FindAndReplace(cwd_str, "", &line);
+    out << line << '\n';
+  }
+}
+
 }  // namespace
 
 int main(int argc, char *argv[]) {
@@ -193,6 +220,7 @@ int main(int argc, char *argv[]) {
 
   std::string linked_binary, dsym_path, bitcode_symbol_map;
   std::string dest_dir;
+  std::string depfile, preprocessed_depfile;
 
   std::unique_ptr<char, decltype(std::free) *> cwd{getcwd(nullptr, 0),
                                                    std::free};
@@ -219,6 +247,10 @@ int main(int argc, char *argv[]) {
     if (SetArgIfFlagPresent(arg, "DEBUG_PREFIX_MAP_PWD", &dest_dir)) {
       arg = "-fdebug-prefix-map=" + std::string(cwd.get()) + "=" + dest_dir;
     }
+    if (SetArgIfFlagPresent(arg, "DEPENDENCY_FILE", &depfile)) {
+      preprocessed_depfile = depfile + "_preprocessed";
+      arg = preprocessed_depfile;
+    }
     if (arg.compare("OSO_PREFIX_MAP_PWD") == 0) {
       arg = "-Wl,-oso_prefix," + std::string(cwd.get()) + "/";
     }
@@ -268,14 +300,15 @@ int main(int argc, char *argv[]) {
     }
   }
 
+  RunSubProcess(processed_args);
+
+  std::string cwd_str = std::string(cwd.get()) + "/";
+  PostProcessDepFile(cwd_str, developer_dir, preprocessed_depfile, depfile);
+
   if (!postprocess) {
-    ExecProcess(processed_args);
-    std::cerr << "ExecProcess should not return. Please fix!\n";
-    abort();
+    return 0;
   }
 
-  RunSubProcess(processed_args);
-
   std::vector<std::string> dsymutil_args = {"/usr/bin/xcrun", "dsymutil",
                                             linked_binary, "-o", dsym_path,
                                             "--flat"};