[CIR] Implement non-virtual thunk generation for multiple inheritance

[lanza]

Stack from ghstack (oldest at bottom):

• [CIR] Implement constant expression member pointer casts #2017
• [CIR] Add support for GNU ifunc attribute #2012
• [CIR] Add support for polymorphic typeid with vtable lookup #2007
• [CIR] Add support for ExtVectorBoolType element assignment, comparisons, and logical NOT #2004
• [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
• [CIR] Implement APValue emission for member function pointers #2016
• [CIR] Implement static variable initialization with guarded init #2015
• [CIR] Implement cast expression classes as l-values #2011
• -> [CIR] Implement non-virtual thunk generation for multiple inheritance #2006

This patch implements complete support for generating non-virtual thunks
in ClangIR for multiple inheritance scenarios. Thunks are small adapter
functions that adjust the 'this' pointer and forward calls to the actual
target function when virtual functions are overridden in derived classes
with multiple base classes.

Implementation details:

1. CIRGenVTables.cpp:

   • Implemented generateThunk() to create thunk function bodies with
     proper scope management (SymTableScopeTy + LexicalScope)
   • Implemented emitCallAndReturnForThunk() to emit the call sequence
   • Implemented setThunkProperties() to set linkage and properties
   • Fixed VTable double-insertion bug in vtable emission

2. CIRGenCXXABI.h/cpp:

   • Added virtual methods performThisAdjustment() and
     performReturnAdjustment() to base class
   • Implemented EmitReturnFromThunk() default implementation

3. CIRGenItaniumCXXABI.cpp:

   • Implemented performThisAdjustment() for non-virtual adjustments
     using cir.ptr_stride operations
   • Implemented performReturnAdjustment() for covariant return types
   • Virtual adjustments marked as NYI with llvm_unreachable

4. CIRGenFunction.h/cpp:

   • Added startThunk(), generateThunk(), emitCallAndReturnForThunk(),
     finishThunk(), and emitMustTailThunk() methods
   • Implemented finishThunk() to properly cleanup after thunk generation

5. CIRGenModule.cpp:

   • Removed assertion blocking thunks in setFunctionAttributes()
   • Fixed insertion point management in vtable creation

6. CIRGenCXX.cpp:

   • Added null checks for GlobalDecl in XRay attributes for thunks

7. Test Updates:

   • Added clang/test/CIR/CodeGen/vtable-thunk.cpp for CIR generation
   • Added clang/test/CIR/Lowering/vtable-thunk.cpp for LLVM lowering
   • Fixed offset expectations (16 bytes for x86_64 layout)
   • Reordered FileCheck patterns to match actual CIR output order

This implementation closely follows CodeGen's approach in
CodeGenVTables.cpp and CodeGenItaniumCXXABI.cpp, with adaptations for
CIR's MLIR-based infrastructure (e.g., location requirements, operation
builders, scope management).

Limitations:

• Virtual adjustments (vcall offsets) not yet implemented
• Musttail thunks for variadic functions not yet implemented
• Return adjustments implemented but not extensively tested

Test Plan:

bin/llvm-lit -v ../../clang/test/CIR/CodeGen/vtable-thunk.cpp
bin/llvm-lit -v ../../clang/test/CIR/Lowering/vtable-thunk.cpp

Both tests pass, verifying:

• CIR generation with correct thunk mangling and operations
• Vtable contains thunk references
• Lowering to LLVM IR produces correct getelementptr operations