Fix Issue 10626 - ICE with vector operation

Fix regression by forwarding buildArrayIdent and buildArrayLoop to the base implementation for all other derived expressions.

Add test cases to improve coverage.
fix Issue 10768 - DMD does not show deprecation message for missing 'override' keyword

If a class/struct/template is not speculatively instantiated, error gagging should be ungagged temporarily during its semantic.
Use RAII idiom to do it reliably.
Improve bug4269 test cases
Merge pull request #2453 from 9rnsr/fix10768

Issue 10768 - DMD does not show deprecation message for missing 'override' keyword
Merge pull request #2345 from yebblies/issue10626

Fix Issue 10626 - ICE with vector operation