[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: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@328384 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang