Build Clang with ThinLTO on Linux

This increases the Linux Clang packaging time from ~1:10 to ~2:30, but is still comfortably faster than the Mac packaging.
This improves the performance of Clang by ~3.5%.

Bug: 1108051
Change-Id: I9cb323ae496df95fd9dd0d22dab4717ea774df88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2327276
Commit-Queue: Arthur Eubanks <aeubanks@google.com>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797939}

tools/clang/scripts/package.py

@@ -184,9 +184,14 @@ def main():
     shutil.rmtree(LLVM_BOOTSTRAP_INSTALL_DIR, ignore_errors=True)
     shutil.rmtree(LLVM_BUILD_DIR, ignore_errors=True)
 
-    build_cmd = [sys.executable, os.path.join(THIS_DIR, 'build.py'),
-                 '--bootstrap', '--disable-asserts',
-                 '--run-tests', '--pgo']
+    build_cmd = [
+        sys.executable,
+        os.path.join(THIS_DIR, 'build.py'), '--bootstrap', '--disable-asserts',
+        '--run-tests', '--pgo'
+    ]
+    if sys.platform.startswith('linux'):
+      build_cmd.append('--thinlto')
+
     TeeCmd(build_cmd, log)
 
   stamp = open(STAMP_FILE).read().rstrip()

tools/clang/scripts/update.py

@@ -39,7 +39,7 @@ import zipfile
 # Reverting problematic clang rolls is safe, though.
 # This is the output of `git describe` and is usable as a commit-ish.
 CLANG_REVISION = 'llvmorg-12-init-1771-g1bd7046e'
-CLANG_SUB_REVISION = 3
+CLANG_SUB_REVISION = 5
 
 PACKAGE_VERSION = '%s-%s' % (CLANG_REVISION, CLANG_SUB_REVISION)
 RELEASE_VERSION = '12.0.0'