Install llvm-ar in the toolchain and use it by default on Unix

A recent SPM change made it mandatory for there to be an archiver in the
toolchain/PATH, swiftlang/swift-package-manager#5761, so make sure llvm-ar is
bundled in the toolchain.

lib/Driver/UnixToolChains.cpp

@@ -398,12 +398,8 @@ toolchains::GenericUnix::constructInvocation(const StaticLinkJobAction &job,
 
   ArgStringList Arguments;
 
-  const char *AR;
+  const char *AR = "llvm-ar";
   // Configure the toolchain.
-  if (getTriple().isAndroid())
-    AR = "llvm-ar";
-  else
-    AR = context.OI.LTOVariant != OutputInfo::LTOKind::None ? "llvm-ar" : "ar";
   Arguments.push_back("crs");
 
   Arguments.push_back(

utils/build-presets.ini

@@ -19,7 +19,7 @@ swift-install-components=back-deployment;compiler;clang-builtin-headers;stdlib;s
 [preset: mixin_buildbot_install_components_with_clang]
 
 swift-install-components=autolink-driver;back-deployment;compiler;clang-resource-dir-symlink;stdlib;sdk-overlay;static-mirror-lib;toolchain-tools;license;sourcekit-xpc-service;swift-remote-mirror;swift-remote-mirror-headers
-llvm-install-components=llvm-cov;llvm-profdata;IndexStore;clang;clang-resource-headers;compiler-rt;clangd;dsymutil;LTO;clang-features-file
+llvm-install-components=llvm-ar;llvm-cov;llvm-profdata;IndexStore;clang;clang-resource-headers;compiler-rt;clangd;dsymutil;LTO;clang-features-file
 
 [preset: mixin_buildbot_trunk_base]
 # Build standard library and SDK overlay for iOS device and simulator.
@@ -800,7 +800,7 @@ no-swift-stdlib-assertions
 [preset: mixin_linux_install_components_with_clang]
 
 swift-install-components=autolink-driver;compiler;clang-resource-dir-symlink;stdlib;swift-remote-mirror;sdk-overlay;static-mirror-lib;toolchain-tools;license;sourcekit-inproc
-llvm-install-components=llvm-cov;llvm-profdata;IndexStore;clang;clang-resource-headers;compiler-rt;clangd;lld;LTO;clang-features-file
+llvm-install-components=llvm-ar;llvm-cov;llvm-profdata;IndexStore;clang;clang-resource-headers;compiler-rt;clangd;lld;LTO;clang-features-file
 
 [preset: mixin_linux_installation]
 mixin-preset=

utils/build_swift/build_swift/defaults.py

@@ -118,7 +118,7 @@ def llvm_install_components():
     """Convenience function for getting the default llvm install components for
     platforms.
     """
-    components = ['llvm-cov', 'llvm-profdata', 'IndexStore', 'clang',
+    components = ['llvm-ar', 'llvm-cov', 'llvm-profdata', 'IndexStore', 'clang',
                   'clang-resource-headers', 'compiler-rt', 'clangd']
     if os.sys.platform == 'darwin':
         components.extend(['dsymutil'])