Need a way to provide system defines and system includes when compiler querying is disabled (with compilerPath of "").

It's possible to disable compiler querying (and disable fallback to a different compiler on failure) by specifying a compilerPath of "". This also works in combination with a custom configuration provider or compile_commands.json, to prevent use of the compiler they indicate (in case it's not cl.exe, gcc, or a gcc variant we can query). However, the defines and includes from c_cpp_properties.json are not currently getting merged into the configuration in those cases. Since no compiler will be queried, it seems necessary to support a way for the user to provide the missing system includes and systems defines.

This issue tracks merging in the includes and defines from c_cpp_properties.json when a compilerPath of "" is used in combination with a custom configuration provider or compile_commands.json.

Trace how c_cpp_properties.json is merged when compilerPath is empty and a custom configuration provider or compile_commands.json supplies configuration. Verify the behavior with configurations where compiler querying is disabled, then ensure the user-provided system includes and defines are present without querying a compiler.