tsan: handle bugs in symbolizer more gracefully

For symbolizer we only process SIGSEGV signals synchronously
(which means bug in symbolizer or in tsan).
But we still want to reset in_symbolizer to fail gracefully.
Symbolizer and user code use different memory allocators,
so if we don't reset in_symbolizer we can get memory allocated
with one being feed with another, which can cause more crashes.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D107564