[ASTImporter] Also import overwritten file buffers

Summary:
Overwritten file buffers are at the moment ignored when importing and
instead only the underlying file buffer is imported.

This patch fixes this by not going to the underlying file entry if the file has
an overwritten buffer.

Reviewers: martong, a.sidorin, shafik

Reviewed By: martong, shafik

Subscribers: rnkovacs

Differential Revision: https://reviews.llvm.org/D78086