[sanitizers] Avoid using -fomit-frame-pointer on Darwin

On Darwin, we're having multiple issues with using -fomit-frame-pointer in the AddressSanitizer and ThreadSanitizer runtimes, so we're actually not using -fomit-frame-pointer in the our builds of the sanitizer dylibs. This patch just pushes our internal change upstream.

The issues are usually with debuggers, profilers and other tools that unwind the stack (crash reporter), which are often simply not able to get a stack trace. And crashlogs that don't contain a stack trace are a huge problem.

Differential Revision: https://reviews.llvm.org/D31376

llvm-svn: 298859