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.