ASan: fix potential use-after-free in backtrace interceptor

Various ASan interceptors may corrupt memory if passed a
pointer to freed memory (https://github.com/google/sanitizers/issues/321).
This patch fixes the issue for the backtrace interceptor,
by calling REAL(backtrace) with a known-good scratch buffer,
and performing an addressability check on the user-provided
buffer prior to writing to it.

Differential Revision: https://reviews.llvm.org/D150496