Filter out the clang-tidy #pragma once in main file warnings

[sean-mcmanus]

          > @H-G-Hristov The `#pragma once in main file` occurs if clang-tidy is run on the header and we're not able to select a source file to associate with it to run on instead. You can get rid of it via adding using setting `"C_Cpp.codeAnalysis.clangTidy.args": ["--extra-arg=-Wno-pragma-once-outside-header"]`, assuming you don't want to fix the source file association. **_Maybe we could filter those warnings out ourselves._**

I'd appreciate that.

Originally posted by @H-G-Hristov in #10463 (comment)

[sean-mcmanus]

Fixed with https://github.com/microsoft/vscode-cpptools/releases/tag/v1.14.4