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://github.com/llvm/llvm-project in the following directories:
  compiler-rt/include/sanitizer
  compiler-rt/lib/sanitizer_common
  compiler-rt/lib/interception
  compiler-rt/lib/asan
  compiler-rt/lib/tsan
  compiler-rt/lib/lsan
  compiler-rt/lib/ubsan
  compiler-rt/lib/hwasan

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.