[CIR] Implement APValue emission for member function pointers

[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 change implements APValue emission for member function pointers in
constexpr contexts, fixing crashes when encountering member function
pointers in constant expressions.

Problem:
Previously, attempting to emit a member function pointer in a constant
expression would hit an assertion at CIRGenExprConst.cpp:2075 with
"not implemented". This prevented compilation of valid C++ code that
uses member function pointers as compile-time constants.

Solution:
The implementation follows CodeGen's approach in CGExprConstant.cpp,
delegating to the CXX ABI for member pointer emission. Key changes:

1. Extended MethodAttr to include an 'adjustment' field for the
   this-pointer adjustment needed for multiple inheritance and virtual
   base classes, per Itanium C++ ABI 2.3.2 (member function pointers are
   represented as struct { fnptr_t ptr; ptrdiff_t adj }).

2. Added CIRGenCXXABI interface methods:

   • emitMemberPointer: Emit a member pointer from an APValue
   • buildMemberFunctionPointer: Build with method and adjustment
   • buildMemberDataPointer: Build data member pointer (currently NYI)

3. Implemented these methods in CIRGenItaniumCXXABI:

   • Virtual methods: Store vtable offset + 1
   • Non-virtual methods: Store function pointer
   • Null pointers: Create null MethodAttr
   • All with proper this-pointer adjustments

4. Updated ConstantEmitter::tryEmitPrivate to delegate member pointer
   emission to the ABI implementation, with proper llvm_unreachable
   guard for NYI derived-to-base conversions

5. Updated lowering in ItaniumCXXABI.cpp to use the adjustment field
   when lowering MethodAttr to LLVM IR

6. Added support in LowerToLLVM.cpp for lowering MethodAttr global
   initializers by delegating to the ABI layer (matching the pattern
   used for DataMemberAttr)

Testing (member-ptr-init.cpp):

• CIR output: Validates #cir.method attributes for non-virtual, virtual,
  and null member function pointers
• LLVM lowering: Validates correct { i64, i64 } struct layout matching
  Itanium ABI with function pointer/vtable offset and adjustment
• OGCG comparison: Confirms CIR's LLVM output exactly matches original
  CodeGen for all test cases

All 385 existing CIR CodeGen tests continue to pass.

Implementation follows ClangIR conventions:

• Copies CodeGen's delegation pattern to CXXABI
• Has tests for CIR output, LLVM lowering, and OGCG comparison
• All NYI code paths properly guarded with llvm_unreachable
• Code formatted with clang-format

[bcardosolopes]

LGTM after nit

[bcardosolopes]

*abiLLVMType