Add {TypeSystem|CompilerType}::GetTypeForFormatters()

Different type system may have different notions of attributes of a type that do not matter for data formatters matching purposes
For instance, in the case of clang types, we remove some qualifiers (e.g. "volatile") as it doesn't make much sense to differentiate volatile T from T in the data formatters

This new API allows each type system to generate, if needed, a type that does not have those unwanted attributes that the data formatters can then consume to generate matches

llvm-svn: 248359