[clang] Update code that assumes FileEntry::getName is absolute NFC

It's an accident that we started return asbolute paths from
FileEntry::getName for all relative paths. Prepare for getName to get
(closer to) return the requested path. Note: conceptually it might make
sense for the dependency scanner to allow relative paths and have the
DependencyConsumer decide if it wants to make them absolute, but we
currently document that it's absolute and I didn't want to change
behaviour here.

Differential Revision: https://reviews.llvm.org/D130934