Move asan-coverage into a separate phase.

Summary:
This change moves asan-coverage instrumentation
into a separate Module pass.
The other part of the change in clang introduces a new flag
-fsanitize-coverage=N.
Another small patch will update tests in compiler-rt.

With this patch no functionality change is expected except for the flag name.
The following changes will make the coverage instrumentation work with tsan/msan

Test Plan: Run regression tests, chromium.

Reviewers: nlewycky, samsonov

Reviewed By: nlewycky, samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6152