[CIR] Add support for ExtVector Bool Type

[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

Implements support for ext_vector_type with bool elements. Bool vectors
are represented as integers in CIR (e.g., bool4 uses !cir.int<u, 8>),
matching traditional CodeGen's approach.

Key changes:

• CIRGenTypes: Convert ExtVectorBoolType to integer storage (iN where N = max(num_elements, 8))
• CIRGenExprConst: Pack bool elements into integer bits during constant initialization
• CIRGenExprScalar: Handle subscript access by extracting bits from integer
• CIRGenExpr: Skip vector optimizations for ExtVectorBoolType in load/store paths

Tests added for basic initialization, subscript access, and bitwise operations.

[bcardosolopes]

In ClangIR, ExtVectorBoolType is always represented as an integer type (!cir.int<u, N>) throughout the IR, including both in registers and in memory. This differs from traditional CodeGen where it may exist as a vector type that needs conversion to integer for storage

I'm not convinced this is how we want to proceed, what are the tradeoffs here?