[Clang] Fix unconditional access to Attr pointer when checking if _Nullable is applicable to a type

In TransformAttributedType(...) when checking if _Nullable can be applied to a
type it dereferences TL.getAttr() unconditionally which we can see from the code
earlier in the function is not correct since it is expected to be nullptr in
some cases.

It looks like the correct course of action is to use TL.getModifiedLoc() over
TL.getAttr()->getLocation() in the case that TL.getAttr() returns a nullptr.

Fixes: https://github.com/llvm/llvm-project/issues/60344

Differential Revision: https://reviews.llvm.org/D142799