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

This reverts commit 1501aabf.

Reason for revert: crbug.com/877063 speculative fix.
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@chromium.org,hans@chromium.org,thomasanderson@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: none
Change-Id: I632e2e47d856d2c7df7e629f12068903e149d166
Reviewed-on: https://chromium-review.googlesource.com/1187344Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585673}

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=2
+CLANG_SUB_REVISION=1
 
 PACKAGE_VERSION = "%s-%s" % (CLANG_REVISION, CLANG_SUB_REVISION)
 
@@ -307,6 +307,18 @@ 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':
@@ -489,6 +501,7 @@ def UpdateClang(args):
       print 'Removing old lib dir: %s' % old_lib_dir
       RmTree(old_lib_dir)
 
+  DownloadHostGcc(args)
   AddCMakeToPath(args)
   AddGnuWinToPath()
 
@@ -501,6 +514,21 @@ 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 = []
@@ -550,6 +578,11 @@ 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