[SanitizerCoverage] Introduce SanitizerCoverageOptions struct.

Summary:
This gives frontend more precise control over collected coverage
information. User can still override these options by passing
-mllvm flags.

No functionality change.

Test Plan: regression test suite.

Reviewers: kcc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9539

llvm-svn: 236687