[llvm-rc] Allow specifying language with a leading 0x prefix

This option is always interpreted strictly as a hexadecimal string,
even if it has no prefix that indicates the number format, hence
the existing call to StringRef::getAsInteger(16, ...).

StringRef::getAsInteger(0, ...) consumes a leading "0x" prefix is
present, but when the radix is specified, the radix shouldn't
be included.

Both MS rc.exe and GNU windres accept the language with that
prefix.

Also allow specifying the codepage to llvm-windres with a different
radix, as GNU windres allows that (but MS rc.exe doesn't).

This fixes https://llvm.org/PR51295.

Differential Revision: https://reviews.llvm.org/D107263