Fix bug 10937 struct inside union gives uninitialized error in CTFE

Remove over-zealous assert, add comments to explain why.
Merge pull request #2567 from donc/regression10994void

Regression 10937 struct inside union gives uninitialized error in CTFE
fix Issue 11039 - Undefined instantiation from circular imports
Fixes Issue 10099 - Better diagnostic for a disabled default constructor.
Merge pull request #2572 from AndrejMitrovic/Fix10099

Issue 10099 - Better diagnostic for a disabled default constructor.
Merge pull request #2556 from WalterBright/fix11039

fix Issue 11039 - Undefined instantiation from circular imports