Build Clang with GCC 10.2.0 on Linux

The LLVM code keeps running into issues with the GCC version we've
been using so far (5.3.0). Try a more recent version to hopefully
get fewer problems.

This was built on a Trusty machine, according to the recipe at
https://bugs.chromium.org/p/chromium/issues/detail?id=929645#c9

Bug: 1164952
Change-Id: If5e015d011417bc5b9669829ceefae3266808e20
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622968
Auto-Submit: Hans Wennborg <hans@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842492}

tools/clang/scripts/build.py

@@ -317,9 +317,9 @@ def MaybeDownloadHostGcc(args):
   """Download a modern GCC host compiler on Linux."""
   if not sys.platform.startswith('linux') or args.gcc_toolchain:
     return
-  gcc_dir = os.path.join(LLVM_BUILD_TOOLS_DIR, 'gcc530trusty')
+  gcc_dir = os.path.join(LLVM_BUILD_TOOLS_DIR, 'gcc-10.2.0-trusty')
   if not os.path.exists(gcc_dir):
-    DownloadAndUnpack(CDS_URL + '/tools/gcc530trusty.tgz', gcc_dir)
+    DownloadAndUnpack(CDS_URL + '/tools/gcc-10.2.0-trusty.tgz', gcc_dir)
   args.gcc_toolchain = gcc_dir