Reland "Clang: stop building with GCC/libstdc++ from Ubuntu Precise (roll 338452-1:338452-2)"

This is a reland of 1501aabf
The goma failures which caused the previous revert are believe to be flaky.

Original change's description:
> Clang: stop building with GCC/libstdc++ from Ubuntu Precise (roll 338452-1:338452-2)
>
> Previously we used a GCC/libstdc++ package from Ubuntu Precise to build a Clang
> that would work on Precise or later. These days, all bots and developers are on
> Trusty or later, so this should no longer be necessary.
>
> Bug: none
> Change-Id: I518ee55d07c04dc89163fe262c81d935e05fed78
> Reviewed-on: https://chromium-review.googlesource.com/1183915
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
> Commit-Queue: Hans Wennborg <hans@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#585067}

TBR=thakis

Bug: none
Change-Id: Ibd2be5064bbdf0ed36aa09c0525825c530465813
Reviewed-on: https://chromium-review.googlesource.com/1185684Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585318}

tools/clang/scripts/update.py

@@ -35,7 +35,7 @@ if use_head_revision:
   CLANG_REVISION = 'HEAD'
 
 # This is incremented when pushing a new build of Clang at the same revision.
-CLANG_SUB_REVISION=1
+CLANG_SUB_REVISION=2
 
 PACKAGE_VERSION = "%s-%s" % (CLANG_REVISION, CLANG_SUB_REVISION)
 
@@ -307,18 +307,6 @@ def CreateChromeToolsShim():
     f.write('endif (CHROMIUM_TOOLS_SRC)\n')
 
 
-def DownloadHostGcc(args):
-  """Downloads gcc 4.8.5 and makes sure args.gcc_toolchain is set."""
-  if not sys.platform.startswith('linux') or args.gcc_toolchain:
-    return
-  gcc_dir = os.path.join(LLVM_BUILD_TOOLS_DIR, 'gcc485precise')
-  if not os.path.exists(gcc_dir):
-    print 'Downloading pre-built GCC 4.8.5...'
-    DownloadAndUnpack(
-        CDS_URL + '/tools/gcc485precise.tgz', LLVM_BUILD_TOOLS_DIR)
-  args.gcc_toolchain = gcc_dir
-
-
 def AddSvnToPathOnWin():
   """Download svn.exe and add it to PATH."""
   if sys.platform != 'win32':
@@ -501,7 +489,6 @@ def UpdateClang(args):
       print 'Removing old lib dir: %s' % old_lib_dir
       RmTree(old_lib_dir)
 
-  DownloadHostGcc(args)
   AddCMakeToPath(args)
   AddGnuWinToPath()
 
@@ -514,21 +501,6 @@ def UpdateClang(args):
 
   cc, cxx = None, None
   libstdcpp = None
-  if args.gcc_toolchain:  # This option is only used on Linux.
-    # Use the specified gcc installation for building.
-    cc = os.path.join(args.gcc_toolchain, 'bin', 'gcc')
-    cxx = os.path.join(args.gcc_toolchain, 'bin', 'g++')
-
-    if not os.access(cc, os.X_OK):
-      print 'Invalid --gcc-toolchain: "%s"' % args.gcc_toolchain
-      print '"%s" does not appear to be valid.' % cc
-      return 1
-
-    # Set LD_LIBRARY_PATH to make auxiliary targets (tablegen, bootstrap
-    # compiler, etc.) find the .so.
-    libstdcpp = subprocess.check_output(
-        [cxx, '-print-file-name=libstdc++.so.6']).rstrip()
-    os.environ['LD_LIBRARY_PATH'] = os.path.dirname(libstdcpp)
 
   cflags = []
   cxxflags = []
@@ -578,11 +550,6 @@ def UpdateClang(args):
       cc = os.path.join(LLVM_BOOTSTRAP_INSTALL_DIR, 'bin', 'clang')
       cxx = os.path.join(LLVM_BOOTSTRAP_INSTALL_DIR, 'bin', 'clang++')
 
-    if args.gcc_toolchain:
-      # Tell the bootstrap compiler to use a specific gcc prefix to search
-      # for standard library headers and shared object files.
-      cflags = ['--gcc-toolchain=' + args.gcc_toolchain]
-      cxxflags = ['--gcc-toolchain=' + args.gcc_toolchain]
     print 'Building final compiler'
 
   # LLVM uses C++11 starting in llvm 3.5. On Linux, this means libstdc++4.7+ is