Fix missing symbols for template alias arguments during codegen

[oCHIKIo]

Fixes #22848

i discovered that the compiler was skipping code generation for symbols (like nested functions) when they were passed only as alias arguments to templates. this resulted in "undefined reference" linker errors because the backend never visited those dependencies.
this PR adds a traversal of template arguments (tiargs) to the toObjFile visitor in toobj.d. this ensures that all symbols required by a non-speculative template instantiation are correctly processed for code generation.

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#22849"

[dkorpel]

A function passed as a template parameter doesn't need code gen when it's not used, that is intentional. In the issue's test case, neither function needs code gen.