[bootstrap] Use separate module cache for builds (#3061)

This will ensure that multiple build jobs running in parallel on the
same machine using different Swift compilers (like it could be the case in CI) do not interfere with
each other.

Addresses rdar://71487295

Author: edymtt

Utilities/bootstrap

@@ -449,6 +449,9 @@ def build_with_cmake(args, cmake_args, source_path, build_dir, targets=[]):
         if args.sysroot:
             swift_flags = "-sdk %s" % args.sysroot
 
+        # Ensure we are not sharing the module cache with concurrent builds in CI
+        swift_flags += ' -module-cache-path "{}"'.format(os.path.join(build_dir, 'module-cache'))
+
         cmd = [
             args.cmake_path, "-G", "Ninja",
             "-DCMAKE_MAKE_PROGRAM=%s" % args.ninja_path,
@@ -775,6 +778,11 @@ def get_swiftpm_flags(args):
     elif cross_compile_hosts:
         error("cannot cross-compile for %s" % cross_compile_hosts)
 
+    # Ensure we are not sharing the module cache with concurrent builds in CI
+    local_module_cache_path=os.path.join(args.build_dir, "module-cache")
+    for modifier in ["-Xswiftc", "-Xmanifest"]:
+        build_flags.extend([modifier, "-module-cache-path", modifier, local_module_cache_path])
+
     return build_flags
 
 if __name__ == '__main__':