[clang] Use `FileEntryRef` in modular header search (part 1/2)

[clang] Use `FileEntryRef` in modular header search (part 1/2)

This patch removes some deprecated uses of `{File,Directory}Entry::getName()`. No functional change indended.

Depends on D151853.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D151854