[WebAssembly] Reapply r319186: "Support bitcasted function addresses with varargs."

This puts the functionality under control of a command-line option which is
off by default to avoid breaking existing setups.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@320197 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed