[CIR] Handle globals with vptr init (#191291)

When a class contains virtual functions but no data members and has a
trivial constructor, global variables of that type are initialized with
a vptr. CIR was incorrectly creating the global variable with the type
of the vtable (an anonymous record) rather than the class type.

When replacing structors with aliases, we were calling a function to
update argument types at the call sites, but this was only necessary
because we initially generated the call using the same incorrect type
that we used for the global. The type correction wasn't implemented
because we hadn't encountered a case where it was needed. Having found
such a case led me to diagnose the problem as above, and I verified that
the same test case compiled without -mconstructor-aliases just failed in
the verifier because we never hit the replacement code. I'm now
convinced that this argument type fixup isn't necessary, so I replaced
the fixup function with an assert.

Assisted-by: Cursor / claude-4.6-opus-high