[TSan] Let the users suppress use-after-free errors using the "race:" suppressions.

If there's a race between a memory access and a free() call in the client program,
it can be reported as a use-after-free (if the access occurs after the free()) or an ordinary race
(if free() occurs after the access).
We've decided to use a single "race:" prefix for both cases instead of introducing a "use-after-free:" one,
because in many cases this allows us to keep a single suppression for both the use-after-free and free-after-use.

This may be misleading if the use-after-free occurs in a non-racy way (e.g. in a single-threaded program).
But normally such bugs shall not be suppressed.

llvm-svn: 187885