[WebAssembly] Support bitcasted function addresses with varargs.

Generalize FixFunctionBitcasts to handle varargs functions. This in
particular fixes the case where clang bitcasts away a varargs when
calling a K&R-style function.

This avoids interacting with tricky ABI details because it operates
at the LLVM IR level before varargs ABI details are exposed.

This fixes PR35385.

llvm-svn: 319186