[WebAssembly] Re-enable main-function signature rewriting

Re-enable the code to rewrite main-function signatures into
"int main(int argc, char *argv[])", but limited to only handling
the case of "int main(void)", so that it doesn't silently strip
an argument in the "int main(int argc, char *argv[], char *envp[])"
case.

This allows main to be called by C startup code, since WebAssembly
requires caller and callee signatures to match, so it can't rely
on passing main a different number of arguments than it expects.

Differential Revision: https://reviews.llvm.org/D57323

llvm-svn: 352479