Move Expression::toCBuffer and helpers into hdrgen.c
Merge pull request #3249 from yebblies/exptocbuffer

[DDMD] [refactor] Move Expression::toCBuffer and helpers into hdrgen.c
Merge pull request #3250 from AndrejMitrovic/FixVCWarning

Make bool -> int conversion explicit to silence VC build warnings.
Fixes Issue 3913 - Emit better diagnostics for invalid enum member lookups.