[libFuzzer] Use OptForFuzzing attribute with -fsanitize=fuzzer.

[libFuzzer] Use OptForFuzzing attribute with -fsanitize=fuzzer.

Summary:
Disables certain CMP optimizations to improve fuzzing signal under -O1
and -O2.

Switches all fuzzer tests to -O2 except for a few leak tests where the
leak is optimized out under -O2.

Reviewers: kcc, vitalybuka

Reviewed By: vitalybuka

Subscribers: cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D44798

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@328384 91177308-0d34-0410-b5e6-96231b3b80d8