[NFC][Sanitizer] Cleanup ASan's GetStackTrace implementation

Cleanup ASan's  __sanitizer::BufferedStackTrace::UnwindImpl (formerly
GetStackTrace) implementation. Start with ASan because it is the most
complex implementation.

GetStackTrace implementations seem to have started out as exact copies
of the original implementation in ASan, but have diverged in subtle
ways. My goal is to parameterize this algorithm (via templating or
callbacks) so we can share the implementation and get rid of the
inversed dependency (sanitizer_common depends on concrete
implementations in asan, ubsan, etc.). This should also help us to avoid
those pesky linker errors caused by undefined, duplicate, and weak
symbols on Windows.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D58861

llvm-svn: 355355