Re-implement Statement::apply in terms of Visitor
fix Issue 12008 - alias this and "unable to resolve forward reference" error
properly C++ const class mangling
fix Issue 12016 - implicit immutable upcast becomes null in CTFE
Merge pull request #3168 from 9rnsr/fix12016

[REG2.065a] Issue 12016 - implicit immutable upcast becomes null in CTFE
fix Issue 12017 - DDoc leaves out the majority of documentation
Mangle long and ulong correctly on 64-bit linux/osx
explicit is_top_level initialization + additional tests

whitespace fix
Merge pull request #3169 from 9rnsr/fix12017

[REG2.065a] Issue 12017 - DDoc leaves out the majority of documentation
Merge pull request #3163 from 9rnsr/fix12008

[REG2.065a] Issue 12008 - alias this and "unable to resolve forward reference" error
Merge pull request #3158 from yebblies/longmangle

[DDMD] Mangle C++ long and ulong correctly on 64-bit linux/osx
Fix Issue 11991 - Implicit catch picks up wrong 'Throwable'
Merge pull request #3147 from yebblies/issue11991

Issue 11991 - Implicit catch picks up wrong 'Throwable'
[Refactoring] clean up Dsymbol::semantic
fix Issue 12023 - template mixin fails within template class
Merge pull request #3174 from 9rnsr/fix12023

[REG2.065a] Issue 12023 - template mixin fails within template class
[cleanup] Tweak existing code around `new TemplateInstance`
fix Issue 12037 - Link-failure with std.numeric.CustomFloat
Merge pull request #3177 from 9rnsr/fix12037

[REG2.065a] Issue 12037 - Link-failure with std.numeric.CustomFloat
fix Issue 12040 - Compiler segfault with circular reference in variable type