Prefer the clang-format in PATH over the one bundled with the extension

[sean-mcmanus]

The right thing to do is to pick the clang-format in PATH over the VSCode one.

Originally posted by @ncihnegn in #2887 (comment)

[sean-mcmanus]

Fixed with https://github.com/microsoft/vscode-cpptools/releases/tag/0.26.3 .