[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