[OpaquePtrs] Deprecate PointerType::getElementType()

This deprecates PointerType::getElementType() in favor of
Type::getPointerElementType(). The motivation is to make it more
apparent when code accesses the pointer element type, because
getElementType() may also also refer to at least
ArrayType::getElementType() and VectorType::getElementType().

Differential Revision: https://reviews.llvm.org/D117885

Author: nikic

llvm/include/llvm/IR/DerivedTypes.h

@@ -667,9 +667,11 @@ class PointerType : public Type {
                                              unsigned AddressSpace) {
     if (PT->isOpaque())
       return get(PT->getContext(), AddressSpace);
-    return get(PT->getElementType(), AddressSpace);
+    return get(PT->PointeeTy, AddressSpace);
   }
 
+  [[deprecated("Pointer element types are deprecated. You can *temporarily* "
+               "use Type::getPointerElementType() instead")]]
   Type *getElementType() const {
     assert(!isOpaque() && "Attempting to get element type of opaque pointer");
     return PointeeTy;