Roll clang 305735:307486.

This includes a change to the compiler that removes the ability to
suppress availability warnings by redeclaring functions. The new way
to suppress warnings is to enclose the function reference in an
"if (@available)" block.

Previous CLs have cleaned up the code base to use the new way of
suppressing warnings (see bug).

Bug: 735328
Change-Id: I340ac5c16df837d13fa5b8885e9f5a8e7bcf8e0c
Reviewed-on: https://chromium-review.googlesource.com/566280
Commit-Queue: Peter Collingbourne <pcc@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486004}

tools/clang/scripts/update.py

@@ -27,14 +27,14 @@ 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 = '305735'
+CLANG_REVISION = '307486'
 
 use_head_revision = 'LLVM_FORCE_HEAD_REVISION' in os.environ
 if use_head_revision:
   CLANG_REVISION = 'HEAD'
 
 # This is incremented when pushing a new build of Clang at the same revision.
-CLANG_SUB_REVISION=3
+CLANG_SUB_REVISION=1
 
 PACKAGE_VERSION = "%s-%s" % (CLANG_REVISION, CLANG_SUB_REVISION)