[CIR] Add support for GNU ifunc attribute

[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 the GNU indirect function (ifunc)
attribute in ClangIR, enabling runtime CPU feature detection and
function dispatch.

Background:
The ifunc attribute is a GNU extension that allows selecting function
implementations at runtime based on CPU capabilities. A resolver
function is called at program startup to return a pointer to the
appropriate implementation.

Implementation:

• Added cir.func.ifunc operation to CIROps.td to represent ifunc
  declarations in CIR with a resolver function reference
• Implemented emitIFuncDefinition() in CIRGenModule to generate
  cir.func.ifunc operations from AST IFuncDecls
• Extended GetOrCreateCIRFunction() to handle ifunc lookups and
  create/retrieve IFuncOp operations
• Updated ReplaceUsesOfNonProtoTypeWithRealFunction() to support
  replacing ifunc operations when prototypes change
• Modified emitDirectCallee() to allow calls through IFuncOp
• Added CallOp verifier support to accept IFuncOp as valid callee
• Implemented CIRToLLVMIFuncOpLowering to lower cir.func.ifunc to
  LLVM dialect IFuncOp, using ptr type for resolver_type parameter

The implementation closely follows the original CodeGen approach for
generating ifunc declarations, adapted to MLIR's operation-based model.

Testing:
Added comprehensive test coverage in clang/test/CIR/CodeGen/ifunc.c
with three scenarios:

1. Basic ifunc with simple resolver
2. Multiple implementations with function pointer typedef
3. Extern declaration followed by ifunc definition

Tests verify:

• CIR emission produces correct cir.func.ifunc operations
• Direct-to-LLVM lowering generates proper ifunc declarations
• Output matches original CodeGen behavior

Test Plan:
$ build/Release/bin/llvm-lit clang/test/CIR/CodeGen/ifunc.c
PASS: Clang :: CIR/CodeGen/ifunc.c (1 of 1)