[Clang] Remove unnecessary Attr.isArgIdent checks.
The MatrixType, ExtVectorType, VectorSize and AddressSpace attributes
have arguments defined as ExprArguments in Attr.td. So their arguments
should never be ArgIdents and the logic to handle this case can be
The logic has been replaced by an assertion to ensure the arguments
are always ArgExpressions
Reviewed By: erichkeane
Differential Revision: https://reviews.llvm.org/D94092