[libFuzzer][MSVC] Disable exceptions in MSVC headers

Summary:
Disable exceptions in MSVC headers using -D_HAS_EXCEPTIONS=0
to silence compiler warning instead of using /Ehsc.

Reviewers: rnk, morehouse, metzman

Reviewed By: rnk, morehouse, metzman

Subscribers: rnk, morehouse, mgorny

Differential Revision: https://reviews.llvm.org/D57119

llvm-svn: 352144