[PATCH] Remove top-level Clang -fsanitize= flags for optional ASan features.

[Kostya Serebryany]

LGTM
This patch will certainly improve the current state of things, but we risk that we'll need to reinstate part of this patch later when we need some
other user-visible flags again. I am ok with such risk.

http://reviews.llvm.org/D4093