Add option to set default header language

[oncipriani]

When .clang-format includes the setting Language: C, clang-format do not format header files (.h) and displays the message Configuration file(s) do(es) not support C++.

Adding // clang-format Language: C to the beginning of the header files solves the problem and the headers are formatted as expected, but this should be considered a workaround.

There should be an option for telling clang-format to treat all header files as C in order to prevent having to add the comment to all header files. In addition, the default language for header files should match the language set in the .clang-format file.

[llvmbot]

@llvm/issue-subscribers-clang-format

Author: Otávio Cipriani (oncipriani)

When `.clang-format` includes the setting `Language: C`, `clang-format` do not format header files (.h) and displays the message `Configuration file(s) do(es) not support C++`.

Adding // clang-format Language: C to the beginning of the header files solves the problem and the headers are formatted as expected, but this should be considered a workaround.

There should be an option for telling clang-format to treat all header files as C in order to prevent having to add the comment to all header files. In addition, the default language for header files should match the language set in the .clang-format file.

[owenca]

Works as expectedConfirmed with 20.1.5:

$ cat > .clang-format
Language: C
$ cat > a.h
#include <stdio.h>
$ clang-format a.h
#include <stdio.h>
$ rm ../.clang-format
$ clang-format a.h
Configuration file(s) do(es) not support C++: /Users/Owen/fork/llvm-project/Debug/.clang-format
$ 

[oncipriani]

I was using version 20.1.3.

E:\Sources\temp>type a.h
#include <stdio.h>

E:\Sources\temp>type .clang-format
---
Language: C

E:\Sources\temp>clang-format.exe a.h
Configuration file(s) do(es) not support C++: E:\Sources\temp\.clang-format

E:\Sources\temp>clang-format.exe --version
clang-format version 20.1.3 (https://github.com/llvm/llvm-project.git 923a5c4f83d2b3675bb88e9fe441daeaa4d69488)