[CIR] Implement constant expression member pointer casts

[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 support for member pointer casts in constant
expressions, specifically handling CK_DerivedToBaseMemberPointer,
CK_BaseToDerivedMemberPointer, and CK_ReinterpretMemberPointer cast kinds.

The implementation closely follows CodeGen's approach in CGExprConstant.cpp
and ItaniumCXXABI.cpp, adapted for CIR's attribute-based constant model.

Implementation details:

1. Added CIRGenModule::GetNonVirtualBaseClassOffset() - Computes the offset
   between derived and base classes as a constant cir::IntAttr, mirroring
   CodeGen's GetNonVirtualBaseClassOffset() which returns llvm::Constant.

2. Added CIRGenCXXABI::getMemberPointerAdjustment() - Calculates the
   adjustment needed for member pointer conversions by determining the
   derived type based on cast direction and delegating to
   GetNonVirtualBaseClassOffset().

3. Added CIRGenCXXABI::EmitMemberPointerConversion() - Base virtual method
   with NYI default, to be overridden by specific ABI implementations.

4. Implemented CIRGenItaniumCXXABI::EmitMemberPointerConversion() for the
   Itanium C++ ABI:

   • CK_ReinterpretMemberPointer: Returns source unchanged (no adjustment)
   • CK_DerivedToBaseMemberPointer/CK_BaseToDerivedMemberPointer:
     • For member data pointers: Adjusts the cir::DataMemberAttr index
       by the offset, matching LLVM's nsw add/sub on constant integers
     • Member function pointers: Marked NYI (requires struct {ptr, adj})

5. Updated ConstExprEmitter::VisitCastExpr() to call the new infrastructure
   instead of llvm_unreachable().

Differences from CodeGen:

• Uses CIR's attribute system (cir::IntAttr, cir::DataMemberAttr) instead
  of LLVM constants
• Member data pointer adjustment directly modifies the DataMemberAttr
  index rather than operating on raw integer constants

Test Plan:

• Added clang/test/CIR/CodeGen/member-pointer-cast.cpp to verify CIR output
• Updated clang/test/CIR/crashes/constexpr-cast.cpp to reflect progress
  (original NYI at line 1006 is fixed; now fails at member function pointer
  emission from APValue, which is a separate feature)

The crash test case from TODO.md item #9 now progresses past the constant
expression cast NYI to a different unimplemented feature (member function
pointer emission), demonstrating that the core cast infrastructure works.

Fixes original NYI at CIRGenExprConst.cpp:1006