[Sanitizer] Fix debug builds of sanitizer_stacktrace_test.cpp
An implementation of __sanitizer::BufferedStackTrace::UnwindImpl is
provided per sanitizer, but there isn't one for sanitizer-common. In
non-optimized builds of the sanitizer-common tests that becomes a problem:
the test sanitizer_stacktrace_test.cpp won't have a reference to that
method optimized away, causing linking errors. This patch provides a dummy
implementation, which fixes those builds.
Differential Revision: https://reviews.llvm.org/D96956