[libFuzzer][MSVC] Use alternatename for ext functions

Summary:
Use alternatename for external functions only when using
MSVC since Clang doesn't support it and MSVC doesn't support
Clang's method (weak aliases).

Reviewers: morehouse

Reviewed By: morehouse

Subscribers: rnk, thakis, mgorny

Differential Revision: https://reviews.llvm.org/D56514

llvm-svn: 351152