Fixes Issue 8041 - __gshared without initializer fails compilation.
Fixes Issue 4479 - Better error message when a module with a name different from its file name is imported via its file name.
Fixes Issue 4528 - Better diagnostic on non-virtual abstract method.
Fixes Issue 9451 - Better diagnostic on unimplemented abstract methods.
Fixup Pull2452 to match interface and abstract function diagnostics.
fix Issue 9650 - __traits(compiles) + mixin

I couldn't minimize test case, but the bug fix is obvious.

The bug had been hiding for a long time, but recent Phobos change (https://github.com/D-Programming-Language/phobos/commit/2a88fea7867f33fd1cf15c2dca7790a3c8aea474) had exposed the bug.

When a mixin expression gets invalid code string, the parsing error had not handled correctly. In such case, it had generated *invalid* AST instead of returning ErrorExp. I guess that the invalid AST would report weird error in later semantic analysis.

I think that both CompileDeclaration and CompileStatement do not have same problem. So they are not changed.
Fixes Issue 9473 - Unittest docs should each be in their own section.
Improve code gen for localgot
Cast T[] to integral type: deprecation -> error.
fix Issue 9723 - add missing main() when compiling exe with -unittest
Merge pull request #1753 from WalterBright/fix9723

fix Issue 9723 - add missing main() when compiling exe with -unittest
convert escape mask literal constant to ESCAPEmask
join declaration and first use for lots of variables
fix Issue 9726 - Add minimum % coverage required for -cov testing
Merge pull request #1639 from AndrejMitrovic/Fix9473

Issue 9473 - Unittest docs should each be in their own section.
Merge pull request #1415 from AndrejMitrovic/Fix4528

Issue 4528 - Better diagnostic on non-virtual abstract method.
Add assertion and fix AST identity that had broken by template instantiation
Merge pull request #1414 from AndrejMitrovic/Fix8041

Issue 8041 - __gshared without initializer fails compilation
Merge pull request #1389 from AndrejMitrovic/Fix4479

Issue 4479 - Better error message with module conflicts
Merge pull request #1620 from AndrejMitrovic/Fix9451

Issue 9451 - Better diagnostic on unimplemented abstract methods.