Imroved default values for well-known LangVersion property o include all currently possible values
Removed concept of single default value since it isn't used now (LangVersion was the last property that actually used it) and it has the same effect as list of default values with a single value in it. So don't repeat ourselves)
LangVersionproperty o include all currently possible valuesLangVersionwas the last property that actually used it) and it has the same effect as list of default values with a single value in it. So don't repeat ourselves)