[Basic] Deprecate MapEntryOptionalStorage::{hasValue,getValue}

MapEntryOptionalStorage is an underlying storage class for
OptionalStorage<clang::DirectoryEntryRef>.

This patch deprecates:

  OptionalStorage<clang::DirectoryEntryRef>::hasValue
  OptionalStorage<clang::DirectoryEntryRef>::getValue

as there is no known users of these two methods.

Differential Revision: https://reviews.llvm.org/D131368