[CIR] Add basic thunk generation infrastructure

[lanza]

Stack from ghstack (oldest at bottom):

• [CIR] Add support for ExtVectorBoolType element assignment, comparisons, and logical NOT #2004
• -> [CIR] Add basic thunk generation infrastructure #2003
• [CIR] Fix RTTI mangling assertion with qualified types #2002
• [CIR] Add support for typeid l-value emission #2001
• [CIR] Fix assertion failure in CIRGenRecordLayout for Empty Base Optimization #1999
• [CIR] Add support for ExtVector Bool Type #1998

This commit adds the foundational support for generating C++ thunks
needed for multiple inheritance, particularly for virtual function calls
through base class pointers. The implementation closely follows CodeGen.

Key changes:

1. Fix VTable double-insertion bug in createGlobalOp/createCIRFunction:

   • Clear insertion point before calling create() to prevent auto-insertion
   • Manually insert operations at correct location
   • This was a pre-existing bug blocking VTable generation during thunk emission

2. Implement thunk generation infrastructure matching CodeGen:

   • Add startThunk() matching CodeGen's StartThunk()
   • Add generateThunk() matching CodeGen's generateThunk()
   • Add emitCallAndReturnForThunk() stub matching CodeGen's EmitCallAndReturnForThunk()
   • Add emitMustTailThunk() stub matching CodeGen's EmitMustTailThunk()
   • Implement finishThunk() matching CodeGen's FinishThunk() for cleanup
   • Enable thunk handling in addVTableComponent() for VTable emission
   • Call generateThunk() from maybeEmitThunk()

3. Make StartFunction null-safe for thunks:

   • Add null checks for GlobalDecl in XRay attributes
   • Guard function body access for thunks (which have no AST body)

4. Handle thunk attributes in setFunctionAttributes:

   • Remove assertion blocking thunks
   • Allow thunks to proceed through attribute setting

5. Add comprehensive test coverage:

   • clang/test/CIR/CodeGen/vtable-thunk.cpp - CIR output test with specific offset verification
   • clang/test/CIR/Lowering/vtable-thunk.cpp - LLVM lowering test with correct offset
   • clang/test/CIR/CodeGen/vtable-thunk-compare-codegen.cpp - CodeGen comparison test (XFAIL until thunk bodies implemented)

Note: Thunk bodies are marked with llvm_unreachable("NYI") placeholders.
Full thunk body emission (this adjustment, call forwarding, return adjustment)
will be implemented in follow-up commits.