[CIR] Implement cast expression classes as l-values

[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 fixes l-value emission for CXXConstCastExpr,
CXXReinterpretCastExpr, CXXFunctionalCastExpr, CXXAddrspaceCastExpr,
and ObjCBridgedCastExpr by routing them through the existing
emitCastLValue() function, matching the behavior of CodeGen.

Previously, these cast expression classes would trigger an error and
assertion when used in l-value contexts (e.g., const_cast<int&>(x) = 1).
This was an artificial limitation - CodeGen routes all cast expression
classes through EmitCastLValue(), which then handles each cast kind
appropriately.

Key insight from CodeGen (CGExpr.cpp:5679-5685):
"Casts are never lvalues unless that cast is to a reference type."

The cast expression class (CXXConstCastExpr vs CXXReinterpretCastExpr)
doesn't determine if it's a valid l-value. Instead, the cast kind
(CK_NoOp, CK_LValueBitCast, etc.) and result type determine validity.
emitCastLValue() already handles this correctly based on getCastKind().

Implementation Details:

• Removed emitError() and assert() for const_cast and related expr classes
• Unified all cast expression classes in emitLValue() switch statement
• All now route through emitCastLValue() like CodeGen (CGExpr.cpp:1823-1832)
• Safety is maintained by emitCastLValue()'s cast kind handling

This follows the ClangIR principle of copying implementations from
CodeGen, which has been proven correct over 20 years.

Testing:

• Single comprehensive test covering const_cast and reinterpret_cast
• Tests verify CIR emission, LLVM lowering, and match with original CodeGen
• All three outputs verified to ensure correctness

Test Plan:
$ ninja -C build/Release clang
$ build/Release/bin/llvm-lit clang/test/CIR/CodeGen/cast-lvalue.cpp