[clang] [llvm] [clang] NFC: Remove `{File, Directory}Entry::getName()` (PR #74910)

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

How about something like this?

```c++
auto FE = CI.getFileManager().getOptionalFileEntryRef("foo.h");
bool IsFooIncluded = FE && CI.getPreprocessor().getIncludedFiles().contains(*FE);
```

https://github.com/llvm/llvm-project/pull/74910