Apple: Fixes for stricter compiler parsing

[dgovil]

Description of Change(s)

An upcoming Clang/LLVM version has stricter mandatory template checking that causes OpenUSD to fail to compile.
These are technically issues in the OpenUSD codebase that go against the C++ standard, but compilers have ignored them thus far. Therefore I am submitting them as fixes here.

We request that these please be part of the 25.2 release to prevent compilation issues for other developers.

The two issues:

1. Clang will check for template uses along uninitialized paths as well during compilation. SdfChildrenProxy had one such path, which had to be removed to allow compilation as it referenced a non existent method.

2. Clang is being stricter about templated function calls in its parser. Sdf_Children<ChildPolicy>::_UpdateChildNames() used a method call that was in violation of the following ISO spec:

   ISO C++03 14.2/4:

   When the name of a member template specialization appears after . or -> in a postfix-expression, or after nested-name-specifier in a qualified-id, and the postfix-expression or qualified-id explicitly depends on a template-parameter (14.6.2), the member template name must be prefixed by the keyword template. Otherwise the name is assumed to name a non-template.

   Explicitly adding the template keyword resolves this. Unfortunately this also gets tagged as unnecessary by older LLVM versions or other compilers. However, a warning felt better than a compile failure.

Checklist

[X] I have created this PR based on the dev branch

[X] I have followed the coding conventions

[ ] I have added unit tests that exercise this functionality (Reference:
testing guidelines)

[X] I have verified that all unit tests pass with the proposed changes

[X] I have submitted a signed Contributor License Agreement (Reference:
Contributor License Agreement instructions)

[jesschimein]

Filed as internal issue #USD-10458

[jesschimein]

/AzurePipelines run

[azure-pipelines]

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[nvmkuruc]

Just to confirm @dgovil -- You're saying that because SdfChildrenProxy doesn't have a _Set method, the assignment operator can never be initialized?

_owner->_Set(*_pos, x);

The change looks good to me, but I'm just making sure I understand which method was non-existent.

[dgovil]

@nvmkuruc Yes, pretty much. In older versions of the compiler, that path would never be taken and so the compiler never discovers that _Set isn't locatable.

However, now it traverses paths that aren't used as a safety measure, and sees that there's no way to find _Set, and so fails to compile.