cmGlobalGenerator: Allow passing language to GetLangaugeOutputExtension

The original GetLanguageOutputExtension took a sourcefile instead of the
name of the language itself. This implementation provided a convenient
handler for when the SourceFile doesn't know what language it is, but
there are times where we know the language, but don't necessarily have a
source file. Adding an overload that takes the name of the language and
returns the extension of that language, or empty string if no extension
is registered.