Changes to c_cpp_properties.json defines/etc. with mergeConfigurations doesn't auto-update until a new configuration is requested

[sean-mcmanus]

1. Set mergeConfigurations to true in c_cpp_properties.json with a configurationProvider set (e.g. "configurationProvider": "ms-vscode.cmake-tools").
2. Change one of the mergeable properties in c_cpp_properties.json like a define in defines.
3. Check the logging or run C/C++: Log Diagnostics.

Bug: The new define/etc. (mergeable property) is not auto-updated until the configuration provider requests a new configuration.