Fix class-implements-interface optional-member crash and add coverage

[ASDAlexander77]

Summary

• Fixes a real crash: class X implements Interface crashed the compiler at DECLARATION time (no instantiation needed) whenever Interface had an optional member (opt?: number) that X didn't declare. mlirGenClassVirtualTableDefinitionForInterface never checked the isMissing flag that getVirtualTable already provides for this legitimate case, unlike its object-literal sibling (mlirGenObjectVirtualTableDefinitionForInterface), which already handles it correctly with a -1 sentinel placeholder. Fixed by mirroring that existing, proven handling.
• Adds cross-module (export/import) test coverage for class+interface combinations with no prior coverage: multilevel interface extends implemented by a class, optional interface members implemented by a class, an abstract class implementing an interface, and structural (no explicit implements) interface satisfaction (a different code path: mlirGenCreateInterfaceVTableForClass, triggered at cast time, vs. the declaration-time mlirGenClassHeritageClauseImplements path).
• Fixes the dynamic-import link failure: a class casting itself (not just a subclass) to one of its own interfaces from the importing module failed to link with "undefined symbol" - mlirGenClassVirtualTableDefinitionForInterface always emitted a constant SymbolRefOp for the implementing method, unlike the class's ordinary vtable builder, which already resolves a dynamic-import-owned method at runtime instead (SearchForAddressOfSymbolOp). Mirroring that fix exposed two further, general (not interface-specific) lowering-pass bugs - both GlobalOpLowering and createFunctionFromRegion were giving a synthesized __cctor constructor function the wrong debug-info location when its initializer is lazily rebuilt from inside an unrelated function - both fixed. This re-enables the multilevel, optional, and structural-interface -shared tests.
• One tier remains disabled: the abstract-class variant surfaces a separate, unrelated bug - an inherited concrete method's internal virtual call (this.area()) returns a wrong value when the method is reached through an interface reference (works correctly via direct or base-class-typed calls). Documented in detail in CMakeLists.txt and left for a dedicated follow-up.

Test plan

• Minimal single-file repro of the optional-member crash - fixed, verified via --emit=obj
• ctest -C Debug -R "implements-interface|structural-interface" - all enabled tiers pass (10/10)
• Full suite: ctest -C Debug -j8 - 786/786 passed, no regressions