[include-cleaner] Handle files with unnamed buffers

Some tools can register virtual buffers without identifiers into the
filemanager. Make sure we can handle pragmas in such cases.

Differential Revision: https://reviews.llvm.org/D157078

GitOrigin-RevId: 31873d3fca38e32a17c2b0aa8975e09aca0f8f89
2 files changed