Do implicit cast for ExpInitializers inside CTFE

Move the implicit cast inside CTFE, so that we're evaluating the
final value.
fix Issue 12138 - Label statement creates an unexpected scope block
fix Issue 11672 - default initialization of static array of structs with a single value fails
fix Issue 5810 - Struct postincrement generates 'no effect' error if used on struct member
Merge pull request #3605 from 9rnsr/fix5810

Issue 5810 - Struct postincrement generates 'no effect' error if used on struct member
Merge pull request #3604 from 9rnsr/fix11672

Issue 11672 - default initialization of static array of structs with a single value fails
Merge pull request #3602 from donc/ctfe_implicit_convert

Do implicit cast for ExpInitializers inside CTFE
Merge pull request #3603 from 9rnsr/fix12138

Issue 12138 - Label statement creates an unexpected scope block