[WebAssembly] Add a pass to create wrappers for function bitcasts.

[WebAssembly] Add a pass to create wrappers for function bitcasts.

WebAssembly requires caller and callee signatures to match exactly. In LLVM,
there are a variety of circumstances where signatures may be mismatched in
practice, and one can bitcast a function address to another type to call it
as that type. This patch adds a pass which replaces bitcasted function
addresses with wrappers to replace the bitcasts.

This doesn't catch everything, but it does match many common cases.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291315 91177308-0d34-0410-b5e6-96231b3b80d8