[Driver][Fuchsia] Support multilib for C++ include dir (llvm#95815)

We generate a separate `__config_site` for each multilib and thus need
to add the additional include dir if it exists.

clang/lib/Driver/ToolChains/Fuchsia.cpp

@@ -433,13 +433,23 @@ void Fuchsia::AddClangCXXStdlibIncludeArgs(const ArgList &DriverArgs,
     if (Version.empty())
       return;
 
-    // First add the per-target include path.
+    // First add the per-target multilib include dir.
+    if (!SelectedMultilibs.empty() && !SelectedMultilibs.back().isDefault()) {
+      const Multilib &M = SelectedMultilibs.back();
+      SmallString<128> TargetDir(Path);
+      llvm::sys::path::append(TargetDir, Target, M.gccSuffix(), "c++", Version);
+      if (getVFS().exists(TargetDir)) {
+        addSystemInclude(DriverArgs, CC1Args, TargetDir);
+      }
+    }
+
+    // Second add the per-target include dir.
     SmallString<128> TargetDir(Path);
     llvm::sys::path::append(TargetDir, Target, "c++", Version);
     if (getVFS().exists(TargetDir))
       addSystemInclude(DriverArgs, CC1Args, TargetDir);
 
-    // Second add the generic one.
+    // Third the generic one.
     SmallString<128> Dir(Path);
     llvm::sys::path::append(Dir, "c++", Version);
     addSystemInclude(DriverArgs, CC1Args, Dir);

clang/test/Driver/fuchsia.cpp

@@ -88,16 +88,16 @@
 // RUN:     -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir \
 // RUN:     -fuse-ld=ld 2>&1\
 // RUN:     | FileCheck %s -check-prefixes=CHECK-MULTILIB-X86
-// RUN: %clangxx -### %s --target=x86_64-unknown-fuchsia -fsanitize=address \
+// RUN: %clangxx -### %s --target=x86_64-unknown-fuchsia -fno-exceptions \
 // RUN:     -ccc-install-dir %S/Inputs/basic_fuchsia_tree/bin \
 // RUN:     -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir \
 // RUN:     -fuse-ld=ld 2>&1\
-// RUN:     | FileCheck %s -check-prefixes=CHECK-MULTILIB-X86,CHECK-MULTILIB-ASAN-X86
-// RUN: %clangxx -### %s --target=x86_64-unknown-fuchsia -fno-exceptions \
+// RUN:     | FileCheck %s -check-prefixes=CHECK-MULTILIB-X86,CHECK-MULTILIB-NOEXCEPT-X86
+// RUN: %clangxx -### %s --target=x86_64-unknown-fuchsia -fsanitize=address \
 // RUN:     -ccc-install-dir %S/Inputs/basic_fuchsia_tree/bin \
 // RUN:     -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir \
 // RUN:     -fuse-ld=ld 2>&1\
-// RUN:     | FileCheck %s -check-prefixes=CHECK-MULTILIB-X86,CHECK-MULTILIB-NOEXCEPT-X86
+// RUN:     | FileCheck %s -check-prefixes=CHECK-MULTILIB-X86,CHECK-MULTILIB-ASAN-X86
 // RUN: %clangxx -### %s --target=x86_64-unknown-fuchsia -fsanitize=address -fno-exceptions \
 // RUN:     -ccc-install-dir %S/Inputs/basic_fuchsia_tree/bin \
 // RUN:     -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir \
@@ -131,8 +131,15 @@
 // RUN:     -fuse-ld=ld 2>&1\
 // RUN:     | FileCheck %s -check-prefixes=CHECK-MULTILIB-X86,CHECK-MULTILIB-COMPAT-X86
 // CHECK-MULTILIB-X86: "-resource-dir" "[[RESOURCE_DIR:[^"]+]]"
-// CHECK-MULTILIB-ASAN-X86: "-L{{.*}}{{/|\\\\}}..{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}asan"
+// CHECK-MULTILIB-NOEXCEPT-X86: "-internal-isystem" "{{.*[/\\]}}include{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}noexcept{{/|\\\\}}c++{{/|\\\\}}v1"
+// CHECK-MULTILIB-ASAN-X86: "-internal-isystem" "{{.*[/\\]}}include{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}asan{{/|\\\\}}c++{{/|\\\\}}v1"
+// CHECK-MULTILIB-ASAN-NOEXCEPT-X86: "-internal-isystem" "{{.*[/\\]}}include{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}asan+noexcept{{/|\\\\}}c++{{/|\\\\}}v1"
+// CHECK-MULTILIB-HWASAN-X86: "-internal-isystem" "{{.*[/\\]}}include{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}hwasan{{/|\\\\}}c++{{/|\\\\}}v1"
+// CHECK-MULTILIB-HWASAN-NOEXCEPT-X86: "-internal-isystem" "{{.*[/\\]}}include{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}hwasan+noexcept{{/|\\\\}}c++{{/|\\\\}}v1"
+// CHECK-MULTILIB-COMPAT-X86: "-internal-isystem" "{{.*[/\\]}}include{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}compat{{/|\\\\}}c++{{/|\\\\}}v1"
+// CHECK-MULTILIB-X86: "-internal-isystem" "{{.*[/\\]}}include{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}c++{{/|\\\\}}v1"
 // CHECK-MULTILIB-NOEXCEPT-X86: "-L{{.*}}{{/|\\\\}}..{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}noexcept"
+// CHECK-MULTILIB-ASAN-X86: "-L{{.*}}{{/|\\\\}}..{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}asan"
 // CHECK-MULTILIB-ASAN-NOEXCEPT-X86: "-L{{.*}}{{/|\\\\}}..{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}asan+noexcept"
 // CHECK-MULTILIB-HWASAN-X86: "-L{{.*}}{{/|\\\\}}..{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}hwasan"
 // CHECK-MULTILIB-HWASAN-NOEXCEPT-X86: "-L{{.*}}{{/|\\\\}}..{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}hwasan+noexcept"