[clang] [clang-format] Fix working -assume-filename with .clang-format-ignore (PR #113100)

[Owen Pan via cfe-commits]

owenca wrote:

Also, the main purpose of `-assume-filename` is to set the language for input from stdin, and IMO the pseudo filename should not be filtered out by any .clang-format-ignore file.

https://github.com/llvm/llvm-project/pull/113100