Update and improve compiler-rt tests for -mllvm -asan_use_after_return=(never|[runtime]|always).

In addition:
  - optionally add global flag to capture compile intent for UAR:
    __asan_detect_use_after_return_always.
    The global is a SANITIZER_WEAK_ATTRIBUTE.

for issue: https://github.com/google/sanitizers/issues/1394

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D103304