[clang][lex] NFCI: Use FileEntryRef in Sema::CodeCompleteIncludedFile

This patch replaces some calls to the deprecated `DirectoryEntry::getName()` with calls to `DirectoryEntryRef::getName()` in SemaCodeComplete.cpp.

Depends on D127654.

Reviewed By: bnbarham

Differential Revision: https://reviews.llvm.org/D127658