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

[via cfe-commits]

kakkoko 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.

It is also worth noting that if the `-style=file` option is specified, it will look for `.clang-format`/`_clang-format` file in the same or parent directory.

Since the specification is such for `.clang-format`/`_clang-format` file, users would naturally expect the same behavior for `.clang-format-ignore` file, wouldn't they?

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