[clangd] Detect C++ language based on well-known file path in vscode extension
Matching the "C++" pattern on the first line of the file doesn't cover
all cases, MSVC C++ headers doesn't have such pattern. This patch
introduce a new heuristic to detect language based on the file path.
MSVC C++ standard headers are in the directory like
"c:\Program Files (x86)\Microsoft Visual Studio\2017\BuildTools\VC\Tools\MSVC\14.15.26726\include"
Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, cfe-commits
Differential Revision: https://reviews.llvm.org/D63483