Add cmake flag LLVM_USE_SANITIZE_COVERAGE

Summary:
When LLVM_USE_SANITIZE_COVERAGE=YES
and one of the sanitizers is used -fsanitize-coverage=3 will be added
to build flag. This will be used to run a coverage-guided fuzzer on various
llvm libraries.

Test Plan: n/a

Reviewers: rnk

Reviewed By: rnk

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7116

llvm-svn: 227216