gcc/libsanitizer

AddressSanitizer and ThreadSanitizer (https://github.com/google/sanitizers) are
projects initially developed by Google Inc.

Both tools consist of a compiler module and a run-time library.
The sources of the run-time library for these projects are hosted at
https://llvm.org/svn/llvm-project/compiler-rt in the following directories:
  include/sanitizer
  lib/sanitizer_common
  lib/interception
  lib/asan
  lib/tsan
  lib/lsan
  lib/ubsan

Trivial and urgent fixes (portability, build fixes, etc.) may go directly to the
GCC tree.  All non-trivial changes, functionality improvements, etc. should go
through the upstream tree first and then be merged back to the GCC tree.
The merges from upstream should be done with the aid of the merge.sh script;
it will also update the file MERGE to contain the upstream revision
we merged with.