[2/2] [MooreToCore] Lower vtable method loads and vptr init

[Scheremo]

Extend class allocation and dispatch lowering to use the concrete vtable
ABI introduced in earlier steps.

This factors out vtable lookup, initializes the object header's vptr when
allocating classes that have a symbolic vtable, and lowers
moore.vtable.load_method into header access, vtable load, slot GEP, and
function-pointer load. Upcasted dispatch remains pointer-preserving and
reuses the concrete object's vptr.

Refine the header GEP sequence to first step through the object header and
then index within the header struct, matching the canonical layout explicitly.

Update MooreToCore tests to check vptr initialization during class.new,
dynamic method slot lookup, indirect dispatch, and dispatch through an
upcasted receiver.

Also fixes a previous issue that would try to call a func.func malloc;
new now uses llvm.func malloc to avoid later linkage issues.

[fabianschuiki]

Results of circt-tests run for e127cd8 compared to results for 1696571:

sv-tests

Changes in emitted diagnostics:

• -6 total change
• -7 error: failed to legalize operation 'moore.vtable.load_method'
• -4 error: failed to legalize operation 'moore.vtable'
• +2 error: 'llvm.return' op operand #0 must be LLVM dialect-compatible type, but got '!sim.queue<i32, 0>'
• -1 error: failed to legalize operation 'func.call'
• +1 error: 'llvm.call' op 'malloc' does not reference a valid LLVM function or IFunc
• +1 error: 'llvm.call' op result #0 must be LLVM dialect-compatible type, but got '!sim.queue<i32, 0>'
• +1 error: Could not resolve vtable for @"class_tb::helloworld"
• +1 error: failed to legalize operation 'moore.fmt.string'