[clang] Add FileEntryRef::getNameAsRequested()

As progress towards having FileManager::getFileRef() return the path
as-requested by default, return a FileEntryRef that can use
getNameAsRequested() to retrieve this path, with the ultimate goal that
this should be the behaviour of getName() and clients should explicitly
request the "external" name if they need to (see comment in
FileManager::getFileRef). For now, getName() continues to return the
external path by looking through the redirects.

For now, the new function is only used in unit tests.

Differential Revision: https://reviews.llvm.org/D131004