cpp: -C and -CC examples
Allow ?: expressions to be const if the selected arm is
Check initialisation from void expressions

Similar to the check about assignment from void.
Added __builtin_choose_expr
cpp: Diagnose unrecognised option correctly
cpp: learnt -CC for stripping no comments
cpp: learnt -C for not stripping non-directive comments
Comment stripping test
Merge branch 'feature/cpp/no-comment-discard'