fix(cpp1): emit requires clause of variable template

[JohelEGP]

Resolves #606.

Testing summary:

100% tests passed, 0 tests failed out of 760

Total Test time (real) =  50.09 sec

Acknowledgements:

• Tested thanks to https://github.com/modern-cmake/cppfront.
• Commit messages follow Conventional Commits 1.0.0.
• Comments and commit messages follow Semantic Line Breaks.

[hsutter]

Thanks! I'll do this, but as a separate commit to avoid rebasing around a conflict with a change I just made for GCC diagnostics.

[hsutter]

Also I'll add the new test case to the existing test file pure2-requires-clauses.cpp2 which already has tests for types and functions, so adding the test for variables belongs there -- I'm trying to keep tests grouped together by area where it makes sense, instead of having single/narrow test cases.