[clang][Sema] Fix PR47676: Handle dependent AltiVec C-style cast

Fix premature decision in the presence of type-dependent expression
operands on whether AltiVec vector initializations from single
expressions are "splat" operations.

Verify that the instantiation is able to determine the correct cast
semantics for both the scalar type and the vector type case.

Note that, because the change only affects the single-expression
case (and the target type is an AltiVec-style vector type), the
replacement of a parenthesized list with a parenthesized expression
does not change the semantics of the program in a program-observable
manner.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D88526