[ADT] Deprecate Optional::getPointer

This patch deprecates Optional::getPointer.  Note that I've converted
all known occurrences of X.getPointer() to &*X on Nov 21, 2022 in
commit 1f914944.

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

Differential Revision: https://reviews.llvm.org/D138621