[FIX][Relax] Preserve closure-referenced private functions

[Yuhx141]

Private functions passed to make_closure still need their original definitions and call signatures. These passes previously updated direct calls only and could remove or replace a function while leaving its closure reference behind.

• retain value-referenced functions in InlinePrivateFunctions;
• retain their tuple parameter signatures in ExpandTupleArguments;
• retain their original parameters and GlobalVar identity in RemoveUnusedParameters.

The three passes share a small internal reference collector. Functions used only by direct calls remain eligible for the existing optimizations.

Closes #20593
Closes #20653
Closes #20654

Tests:

• All three added closure regressions fail on unmodified main.
• pytest tests/python/relax/test_transform_inline_private_functions.py tests/python/relax/test_transform_expand_tuple_args.py tests/python/relax/test_transform_remove_unused_parameters.py -q: 12 passed, including the existing direct-call and recursion controls.
• Release/LLVM build completed for the three modified C++ passes.
• Pre-commit passed on all seven PR files.