[Support] Return an enum instead of an unsigned; NFC.

We seem to be (logically) returning ArchExtKinds here in all cases, so
the return type should reflect that.

The static_cast is necessary because `A.ID` is actually an `unsigned`,
presumably since we use `decltype(A)` to represent extended attributes
for both ARM and AArch64, which use distinct `ArchExtKinds`.

We can't trivially make the same change for ARM, because one of the
values it returns is the bitwise-or of two `ARM::ArchExtKind`s.

llvm-svn: 322613