AST: Replace TypeArrayView<GenericTypeParamType> with ArrayRef<GenericTypeParamType *>
This basically undoes 3da6fe9c0d48, which in hindsight was wrong.
There were no other usages of TypeArrayView anywhere else except for
GenericSignature::getGenericParams(), and it was almost never what
you want, so callers had to convert back and forth to an ArrayRef.
Remove it.