COFF: Fix entry name inference for x86.

Entry name selection rule is already complicated on x64, but it's more
complicated on x86 because of the underscore name mangling scheme.

If one of _main, _main@<number> (a C function) or ?main@@... (a C++ function)
is defined, entry name is _mainCRTStartup. If _wmain, _wmain@<number or
?wmain@@... is defined, entry name is _wmainCRTStartup. And so on.

llvm-svn: 242110