Revert "Roll clang 373424-64a362e7-1 : n330255-55c223a7-1."

This reverts commit 263061fc.

Reason for revert: Causing all builds to fail on mac-builder-perf (https://ci.chromium.org/p/chrome/builders/ci/mac-builder-perf)

Original change's description:
> Roll clang 373424-64a362e7-1 : n330255-55c223a7-1.
> 
> Since upstream llvm/clang has moved to Github, there are no longer any svn revisions.
> Instead, our package versioning scheme now begins with the number of commits since
> the initial commit in the repository, prefixed by an "n".
> 
> These "n" numbers are NOT comparable to svn revisions.
> 
> Ran `tools/clang/scripts/upload_revision.py 55c223a7ed522293cf9995d07d348368c345d1f2`.
> 
> Bug: 1011815
> Change-Id: If2b9dce272182ca4f0881424e1d64410b796b4c4
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879890
> Commit-Queue: Nico Weber <thakis@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#710354}

TBR=thakis@chromium.org,hans@chromium.org

Change-Id: I203d5f8c9e3880d2bc85bb5efa422a9654736a6f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1011815, 1019517
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1889211Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710546}

build/config/compiler/BUILD.gn

@@ -1529,18 +1529,21 @@ config("default_warnings") {
 
           # TODO(https://crbug.com/1002945): Evaluate and possibly enable.
           "-Wno-sizeof-array-div",
+        ]
 
+        cflags_c += [
+          # TODO(https://crbug.com/995993): Clean up and enable.
+          "-Wno-implicit-fallthrough",
+        ]
+        if (llvm_force_head_revision) {
+          cflags += [
             # TODO(https://crbug.com/1016947) Clean up, enable.
             "-Wno-bitwise-conditional-parentheses",
 
             # TODO(https://crbug.com/1016945) Clean up, enable.
             "-Wno-builtin-assume-aligned-alignment",
           ]
-
-        cflags_c += [
-          # TODO(https://crbug.com/995993): Clean up and enable.
-          "-Wno-implicit-fallthrough",
-        ]
+        }
       }
     }
   }

tools/clang/scripts/update.py

@@ -37,8 +37,8 @@ import zipfile
 # Do NOT CHANGE this if you don't know what you're doing -- see
 # https://chromium.googlesource.com/chromium/src/+/master/docs/updating_clang.md
 # Reverting problematic clang rolls is safe, though.
-CLANG_REVISION = '55c223a7ed522293cf9995d07d348368c345d1f2'
-CLANG_SVN_REVISION = 'n330255'
+CLANG_REVISION = '64a362e7216a43e3ad44e50a89265e72aeb14294'
+CLANG_SVN_REVISION = '373424'
 CLANG_SUB_REVISION = 1
 
 PACKAGE_VERSION = '%s-%s-%s' % (CLANG_SVN_REVISION, CLANG_REVISION[:8],