[Bugfix][Relax] Avoid dangling globals in AllocateWorkspace

[Nanmur]

Why

AllocateWorkspace skips rewriting Codegen and Composite function bodies, but it could still rename and remove workspace-bearing global functions referenced by those bodies. This left the skipped callers with dangling GlobalVar references.

What

• Preserve workspace-bearing globals referenced by skipped Codegen/Composite functions.
• Keep the existing workspace rewrite for callees whose call sites can be updated.
• Add a regression test covering main -> Codegen -> global Composite.

Fixes #20479

Testing

• Native incremental build on Windows with MSVC and LLVM enabled.
• python -m pytest tests/python/relax/test_transform_allocate_workspace.py -q (2 passed)
• python tests/lint/check_file_type.py
• git diff --check

[wwoosshh]

I checked this on top of current main (d55759e). The C++ change merges and builds cleanly, but the new test module uses T.Buffer, which was renamed to T.Tensor in #20519. Because NestedGlobalFunctions is defined at module level, tests/python/relax/test_transform_allocate_workspace.py now fails at collection with AttributeError: No script namespace 'Buffer', so none of the tests in that file run.

With the two T.Buffer in add_one replaced by T.Tensor, both tests in the file pass, and test_nested_global_function_is_well_formed fails without the C++ change (the well-formed check after AllocateWorkspace raises), so it covers the fix.

[Nanmur]

Updated in 506d79cd26.

The branch is now rebased onto d55759ec1, and the two add_one parameters use T.Tensor instead of the removed T.Buffer spelling. The C++ fix is unchanged.

Local verification completed:

• the updated test module passes py_compile;
• all repository pre-commit hooks for both changed files pass.

A fresh upstream CI run is starting for the rebased revision. Your confirmation that both focused tests pass and that the regression test fails without the C++ change verifies that the test still exercises the intended bug.

[wwoosshh]

Thanks for the update. I re-checked 506d79cd2 on top of main (d55759e): both tests in tests/python/relax/test_transform_allocate_workspace.py pass, test_nested_global_function_is_well_formed fails when allocate_workspace.cc is reverted to main, and pre-commit passes on the changed files.

LGTM.