CCLS detects my C headers as C++ headers

[poly2it]

CCLS detects my C headers as C++ headers.

Steps to reproduce

1. Create a C project.
2. Create a header file which doesn't have an associated C file.
3. Observe.

System information

• ccls version: 0.20230717
• clang version: 17.0.6
• OS: GNU/Linux
• Editor: Neovim