Extension does not correctly handle WSL symlinks

Type: LanguageService

Describe the bug

• OS and Version: 1803
• WSL version: Ubuntu 18.04
• VS Code Version: 1.24.1
• C/C++ Extension Version: 0.17.4
• Other extensions you installed (and if the issue persists after disabling them):
• A clear and concise description of what the bug is.
  I want to include LLVM in a project but vscode-cpptools seems to fail to intellisense.

To Reproduce
Steps to reproduce the behavior:
c_cpp_prperties.json

{
    "configurations": [
        {
            "name": "WSL",
            "intelliSenseMode": "clang-x64",
            "compilerPath": "/usr/bin/gcc",
            "includePath": [
                "${workspaceFolder}",
                "/mnt/c/libraries/lib1/include",
                "C:/libraries/lib2/include"
            ],
            "defines": [],
            "browse": {
                "path": [
                    "${workspaceFolder}",
                    "/mnt/c/libraries"
                ],
                "limitSymbolsToIncludedHeaders": true,
                "databaseFilename": ""
            },
            "cStandard": "c11",
            "cppStandard": "c++17"
        }
    ],
    "version": 4
}

edit an C/C++ source code file and try to type this

#include <llvm/IR/Module.h>

Expected behavior
show no error read lines and make correct intellisense

Screenshots

Additional context
it seems that the extension do find the LLVM folder but somehow it cannot recognize it. I tried to make a soft link named "llvm" for "llvm-6.0" in /usr/include/ but it is useless.

Reproduce the report using the supplied c_cpp_properties.json, the WSL configuration, and the #include <llvm/IR/Module.h> example. Start by examining how the extension handles the listed Windows and WSL paths and the /usr/include/llvm symlink; done means LLVM headers resolve without errors and IntelliSense works correctly.