ASan: add backtrace_symbols test and clarify code is correct

This is another patch for https://github.com/google/sanitizers/issues/321
(sanitizer interceptors can write to freed memory, causing corruption),
in this case for backtrace_symbols.

backtrace_symbols is already correct, hence this patch removes the
TODO note. Additionally, this patch adds a test case for it.

Differential Revision: https://reviews.llvm.org/D150498