fix Issue 13479 - Templates not emitted when instantiated in mixins mixed in functions

In the PR #3948, I changed the position where `TemplateInstance`s are inserted.

https://github.com/D-Programming-Language/dmd/pull/3948/files#diff-0477a1d81a6a920c99362954179c59c8R5937

But, I didn't notice that glue-layer does not handle `TemplateInstance`s in local members.
Merge pull request #3991 from 9rnsr/fix13479

[REG2.067a] Issue 13479 - Templates not emitted when instantiated in mixins mixed in functions