Sema: Add a new test to record some broken behavior
Add a new test file with some mechanically generated test cases for
various weird combinations of literal type variables joined by
conversion constraints.
The code responsible for this needs an overhaul, and to make sure
we're making forward progress I'm codifying the current behavior
in tests.
We have a handful of other test cases for this throughout the
test suite, I'll try to gradually consolidate them here as well.
