fix Issue 12131 - Struct assign purity error
[cleanup] fix indent style
fix Issue 2025 - Inconsistent rules for instantiating templates with a tuple parameter

1. Adjust matching level for issue cases.

 a. `TemplateTupleParameter::matchArg` should always return `MATCHconvert` in order to have the lower priority than other non-tuple templates.
 b. `TemplateTypeParameter::matchArg` and `TemplateAliasParameter::matchArg` should return same match level (== MATCHconvert) for aggregate types.
 c. In leastAsSpecialized comparison, aggregate types should prefer template type parameter. For that, `TemplateAliasParameter::matchArg` should match to `tdummy`.

2. Add "dependent template parameter" concept

 Their `matchArg` should return binary result - match or nomatch, in order to follow the match level of the corresponding precedent parameter.
fix Issue 1659 - template alias parameters are chosen over all but exact matches.
fix Issue 12077 - Instantiated type does not match to the specialized alias parameter
fix Issue 12211 - Assignment expression is not an lvalue
Give users some guidance on how to handle missing default cases.
Move Struct/ClassDeclaration::toDt into free functions
Move ClassReferenceExp::toDt* into free functions
Merge pull request #3301 from 9rnsr/fix12131

Issue 12131 - Struct assign purity error
Merge pull request #3318 from yebblies/classtodt

[DDMD] [refactor] Move remaining toDt methods into free functions
[cleanup] Tweak style of runnable/mangle.d
[cleanup] Gather `mangleof` property related tests into runnable/mangle.d
[cleanup] Adjust test results in runnable/mangle.d
Move *Declaration::toDebug into free functions
Do not put comments between the if body and 'else'
Merge pull request #3320 from 9rnsr/fix_mangle

[cleanup] Gather `mangleof` property related tests into runnable/mangle.d
Merge pull request #3322 from yebblies/todebug

[DDMD] [refactor] Move *Declaration::toDebug into free functions
Move Dsymbol::toSymbol into a visitor class
Merge pull request #3321 from yebblies/bodycomment

[DDMD] Do not put comments between the if body and 'else'