Introduce -fsanitize-coverage=N flag

Summary:
This change makes the asan-coverge (formerly -mllvm -asan-coverge)
accessible via a clang flag.
Companion patch to LLVM is http://reviews.llvm.org/D6152

Test Plan: regression tests, chromium

Reviewers: samsonov

Reviewed By: samsonov

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6153
[asan] fix coverage tests to use the new flag syntax (-fsanitize-coverage=N)