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

[via cfe-commits]

kakkoko wrote:

@mydeveloperday 

> However what you are saying is, its not using that assume-filename to locate the .clang-format-ignore file?
> 
> Hence in the IDE it reformats myfile.* meaning you can't save it unformatted

Yes, that is correct.

You can see that in [the following file of `Clang-Format.cpp`, `main()`](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.2/clang/tools/clang-format/ClangFormat.cpp#L709-L735):

```c++
  // If no file name is specified on the command line to be processed,
  // formatting is performed on standard input and terminates.
  if (FileNames.empty())
    return clang::format::format("-", FailOnIncompleteFormat);

  ...

  // Sequentially process each file specified on the command line
  for (const auto &FileName : FileNames) {
    // Check for .clang-format-ignore
    const bool Ignored = isIgnored(FileName);
    ...
    // If the pattern matches the pattern written in .clang-format-ignore,
    // do nothing and proceed to the next file
    if (Ignored)
      continue;
    ...
    // do formatting
    Error |= clang::format::format(FileName, FailOnIncompleteFormat);
  }
```

The `isIgnored()` function, which determines whether or not to ignore using `.clang-format-ignore`, is only called in the part that processes the files enumerated on the command line in order.


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