fix Issue 13417 - segmentation fault when deduce template type
fix Issue 13424 - Initialization of delegate to do-nothing default causes segfault at runtime
Merge pull request #3950 from 9rnsr/fix13417

[REG2.066] Issue 13417 - segmentation fault when deduce template type
Merge pull request #3951 from 9rnsr/fix13424

[REG2.066] Issue 13424 - Initialization of delegate to do-nothing default causes segfault at runtime
Remove un-convertable comment
Do not rely on implicit narrowing integral conversions
Merge pull request #3955 from yebblies/comment4

[DDMD] Remove un-convertable comment
Merge pull request #3954 from yebblies/impconv

[DDMD] Do not rely on implicit narrowing integral conversions
Add TemplateInstance::tnext to construct instantiation dependency graph

To detect speculative instantiations in codegen phase, `tnext` is neccesary for the `test6()` case `runnable/link13350.d`. That's a big difference from the PR #2550.
[Refactoring] Do not assert TypeFunction::inuse in mangleFuncType
Remove inferred attributes from mangled names of instantiated functions
Remove workaround for issue 13394 and issue 10329

Issue 13394 - invariant fail to link since 2.066 when using separate compilation
  PR #3918 (f6c130e66791ba9eb047334dec0645d06e74cc89)

Issue 10329 - Attributes not inferred for indirectly templated methods
  PR #2832 (ebae884d9cd3a0c4ebf3c1b129654ba31ee0e73f)

Because attribute inference does not cause link-failure anymore.
Skip TypeInfo code generation of non-root user types
Support -allinst for old TypeInfo generation strategy