[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
removed.

The logic has been replaced by an assertion to ensure the arguments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