Don't use Optional::getPointer (NFC)

Since std::optional does not offer getPointer(), this patch replaces
X.getPointer() with &*X to make the migration from llvm::Optional to
std::optional easier.

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/D138466