[clang-format] .clang-format-ignore ignores -assume-filename option

[kakkoko]

The filename given by the -assume-filename option is used to search for .clang-format files, etc., but is not used to match the contents of the .clang-format-ignore file.

For example, the C/C++ extension of VSCode uses the -assume-filename option to provide a filename, and the content to be formatted is provided from standard input. Therefore, the .clang-format-ignore file will not work for that extension.

[kakkoko]

If "ignore" is implemented as "do not output anything," the formatting result will be empty (and the C/C++ extension interprets the formatting as a failure), so appropriate processing such as outputting standard input directly to standard output is necessary.

Edited: what I wrote above was incorrect. The extension will work properly even if it outputs nothing.