A cache is typically a use case for mutable (as the outside user of the
class wouldn't directly rely on the cache as part of the API, this would
only function to assist the API perform better), so we can mark the
selection cache as mutable and make the code nicer to read overall.
Normalizes the headers for the cpp files in the RuntimeCommon target.
Now all headers and source files within the RuntimeCommon/RuntimeCommonB
targets are normalized and consistent with one another.