Attributes accepting an EnumArgument are allowed to pass a string literal, or an identifier. VariadicEnumArguments now behave consistently instead of only accepting a string literal.

This change affects the only attribute accepting a variadic enumeration: callable_when.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224582 91177308-0d34-0410-b5e6-96231b3b80d8