[clang] Only modify FileEntryRef names that are externally remapped

As progress towards having FileEntryRef contain the requested name of
the file, this commit narrows the "remap" hack to only apply to paths
that were remapped to an external contents path by a VFS. That was
always the original intent of this code, and the fact it was making
relative paths absolute was an unintended side effect.

Differential Revision: https://reviews.llvm.org/D130935