[Driver] Use llvm-ar by default on Unix and copy it over into the build directory

Now that llvm-ar is installed by default in the toolchain, #62510, and a recent
SPM change requires there to be an archiver in the toolchain/PATH,
swiftlang/swift-package-manager#5761, use that bundled llvm-ar for all Unix
platforms, which requires copying it over into the build directory too before
building the corelibs.

Author: finagolfin

Diff for: 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(

Diff for: utils/CMakeLists.txt

@@ -15,3 +15,12 @@ file(TO_CMAKE_PATH "${LLVM_BUILD_BINARY_DIR}/bin/FileCheck${CMAKE_EXECUTABLE_SUF
 swift_install_in_component(PROGRAMS "${_SWIFT_UTILS_FILECHECK}"
                            DESTINATION bin
                            COMPONENT llvm-toolchain-dev-tools)
+
+# Copy over llvm-ar to use in building the Swift portions of the corelibs on
+# Unix platforms.
+if(NOT "${SWIFT_HOST_VARIANT_SDK}" MATCHES "OSX|WINDOWS")
+  file(COPY ${LLVM_RUNTIME_OUTPUT_INTDIR}/bin/llvm-ar
+    DESTINATION "${SWIFT_RUNTIME_OUTPUT_INTDIR}"
+    FILE_PERMISSIONS OWNER_READ OWNER_WRITE OWNER_EXECUTE GROUP_READ
+    GROUP_EXECUTE WORLD_READ WORLD_EXECUTE)
+endif()