minor refactor to improve comments and error recovery
Merge pull request #4071 from WalterBright/exp-refactor

minor refactor to improve comments and error recovery
do not generate __fieldPostBlit if this(this) annotated with @disable
@disable __fieldPostBlit if any of postblits[] are disabled.
Merge pull request #4073 from IgorStepanov/disiabled-field-postblit

do not generate __fieldPostBlit if this(this) annotated with @disable