Please add env/envFile support for LanguageServer

Type: General
Describe the bug

• OS and Version: Ubuntu 19.10
• VS Code Version: 1.41.0
• C/C++ Extension Version: 0.26.2

Related issues: #4691, #4178, #3078

Description
It is common that compiler requires extra environment variables to run correctly (PATH, LD_LIBRARY_PATH, even less-known CPATH in my case) which are provided by some kind of "activation script".

Without proper setting of such variables detecting compiler's default include paths and defines doesn't work (or doesn't work as expected).

Proposal
env / envFile settings are getting more and more wide-spread and it would be awesome if cpptools respects them when detecting compiler and its settings (even via compile_commands.json).
Having such option per-configuration and per-project significantly improves experience when developer needs to switch between different compilers.

Unfortunately I couldn't find place in the repo where compiler is invoked...

Beginne damit, den Compiler-Erkennungspfad von LanguageServer und dessen Verarbeitung von compile_commands.json zu lokalisieren; im Issue werden keine Repository-Dateien oder Tests genannt. Vergleiche das angeforderte Verhalten von env/envFile pro Konfiguration und pro Projekt mit den vorhandenen Einstellungen, und definiere als done, dass die Standard-Include-Pfade und Defines des Compilers in aktivierten Umgebungen korrekt erkannt werden.