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
