[CIR] Add support for polymorphic typeid with vtable lookup

[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 commit implements runtime type identification for polymorphic types,
completing typeid operator support in ClangIR.

Previously, only non-polymorphic (static) typeid was supported. This adds:

1. Polymorphic typeid emission: When typeid is applied to an expression
   of polymorphic class type, we now emit vtable lookup to get the runtime
   type_info pointer.

2. Null pointer checking: For pointer operands, we emit null checks and
   call emitBadTypeidCall (currently unreachable until exception support).
   Matches CodeGen behavior of always checking, even for references.

3. Type safety checks: emitTypeCheck ensures typeid is not used unsafely
   during object construction or destruction.

4. Vtable layouts: Supports both traditional (vtable[-1]) and relative
   (load.relative(vtable, -4)) layouts for Itanium ABI.

Implementation follows CodeGen closely:

• emitTypeidFromVTable handles the main logic with null checking
• CIRGenItaniumCXXABI::emitTypeid handles ABI-specific vtable access
• Uses existing getVTablePtr infrastructure
• shouldTypeidBeNullChecked matches CodeGen (always returns true)

The vtable layout for type_info access:

• Absolute: type_info* at vtable[-1] (8 bytes before vptr)
• Relative: type_info offset at vtable[-4] (4 bytes before vptr)

Test Plan:

• Added typeid-polymorphic.cpp with comprehensive tests
• Tests verify CIR output, LLVM lowering, and CodeGen comparison (OGCG)
• All 4 test cases have CIR + LLVM + OGCG checks
• Covers: basic usage, references, derived classes, const pointers
• Note: CIR lowering of PtrStrideOp does not emit 'inbounds' flag
  (general CIR lowering issue, not specific to this feature)
• All CIR tests pass