[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

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;