Dump Cmm with -ddump-cmm when building .cmm files
Fixes #16131 (closed). Motivation is as listed in the ticket:
- When building Cmm even the parser does code generation, so the Cmm after parsing is quite different than the input and contains useful information.
- I want all macros to be expanded, running CPP manually is painful.
- I currently can't see output of Cmm optimisations when building Cmm files.