[clang][Interp][NFC] Redo returning values from builtin functions

Instead of having retInt/retLong/retSizeT/etc., just add retInteger,
which takes an APSInt and returns it in form of the given QualType.
This makes the code a little neater, but is also necessary since
some builtins have a different return type with -fms-extensions.

Author: tbaederr