[WebAssemby] Support main functions with alternate signatures.

[WebAssemby] Support main functions with alternate signatures.

WebAssembly requires caller and callee signatures to match, so the usual
C runtime trick of calling main and having it just work regardless of
whether main is defined as '()' or '(int argc, char *argv[])' doesn't
work. Extend the FixFunctionBitcasts pass to rewrite main to use the
latter form.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@320041 91177308-0d34-0410-b5e6-96231b3b80d8