[libFuzzer] Disable experimental clang coverage support by default.

Summary:
It can be enabled via "-use_clang_coverage=1" flag. Reason for disabling:
libFuzzer resets Clang Counters and makes it impossible to generate coverage
report for a regular fuzz target (i.e. not standalone build).

Reviewers: kcc

Reviewed By: kcc

Subscribers: kcc

Differential Revision: https://reviews.llvm.org/D38604

llvm-svn: 315029
GitOrigin-RevId: 330496c3fcdff2a429211ed9d9c1cf9795ee6339
5 files changed