Add option to disable the extra clang flags in MacOS [Feature Request]

[Azure-Vani]

In particular, the if add_extra_clang_flags: branch in PrepareFlagsForClang function in file ycmd/ycmd/completers/cpp/flags.py. Otherwise, the result when complete headers is super noisy when somebody uses g++ rather than clang on macOS

[bstaletic]

I don't think that is a good idea. That branch that reaches into AddMacIncludePaths is also responsible for adding -resource-dir= and -fno-delayed-template-parsing on Windows. I've done invasive changes in flags.py before and it was very frustrating.

That said, you can still avoid that by:

• Switching to the clangd based completer
• Using compilation databases instead of the extra conf files.

[puremourning]

I actually had reason to need this recently myself for some very bizarre reasons so maybe I will add it at some point.