IntelliSense-based features don't properly distinguish between -isystem and -iquote include paths

[sean-mcmanus]

We only send IntelliSense a single list of include paths instead of separate "quote" and "system" paths.

Repro is like

#include <string>

int main()
{
    std::string s;
    return 0;
}

with

           "compilerPath": "C:/Program Files/LLVM/bin/clang++.exe",
            "compilerArgs": [
                "-iquote${workspaceFolder}/f1"
            ]

where f1 has a file named string.

It incorrectly picks up the -iquote path for <>.

Originally reported at #12198

[sean-mcmanus]

Fixed with https://github.com/microsoft/vscode-cpptools/releases/tag/v1.21.0