[clang-format] Assert that filenames are not empty

Summary:
Adds asserts to catch empty filenames, which otherwise will cause a crash in SourceManager.
The clang-format tool now outputs an error if an empty filename is used.
Fixes bug: 34667

Reviewers: krasimir, djasper, MyDeveloperDay

Reviewed By: MyDeveloperDay

Subscribers: cfe-commits

Patch by: @jr

Tags: #clang-format, #clang

Differential Revision: https://reviews.llvm.org/D56345