sanitizers: turn thread-safety errors into warnings

There were 2 issues reported on https://reviews.llvm.org/D105716:
1. FreeBSD phtread.h is annotated with thread-safety attributes
and this causes errors in gtest headers.
2. If sanitizers are compiled with an older versions of clang
(which supports the annotations, but has some false positives
in analysis not present in later versions of clang), compilation
fails with errors.

Switch the errors to warnings by default.
Some CI bots enable COMPILER_RT_ENABLE_WERROR, which should
turn these warnings back into errors.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D107826