Metal support: clangd is incorrectly invoked for Metal shader files

Qt Creator 10.0 currently treats Metal shader source files as C++ using the clangd code model.

However, Apple's Metal shaders are written using nonstandard extensions to C++14, and are compiled using a special variant of the clang/LLVM toolchain in a different environment.

This results in a large number of incorrect warnings when editing a *.metal shader.

Qt Creator should use an appropriately configured clangd code model for Metal shader source files.