Header-only fallback incorrectly occurs when system headers are opened using compileCommands

Use test.cpp

#include <string>

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

with some compile_commands.json like

[
    {
        "directory": "<dir>",
        "command": "C:\\PROGRA~2\\MICROS~3\\2017\\ENTERP~1\\VC\\Tools\\MSVC\\1416~1.270\\bin\\Hostx86\\x86\\cl.exe -c sub.cpp",
        "file": "sub.cpp"
    }
]

Then use Go to Definition on std::string.

Bug: The TU for xstring gets opened as a header only fallback instead of using the existing TU for sub.cpp.

This bug doesn't repro when using the CMake Tools configurationProvider.

Bug2 in https://github.com/microsoft/vscode-cpptools/issues/5506 is related.

Reproduce the issue with test.cpp, the shown compile_commands.json entry for sub.cpp, and Go to Definition on std::string. Trace why xstring is opened as a header-only fallback instead of using the existing sub.cpp translation unit; done means the compile_commands path resolves the existing TU, matching the CMake Tools configurationProvider behavior.