[CIR] Implement static variable initialization with guarded init

[lanza]

Stack from ghstack (oldest at bottom):

• -> [CIR] Implement static variable initialization with guarded init #2045

Add support for C++ static local variables with non-trivial destructors
and dynamic initialization. This implements the guarded initialization
pattern required by the C++ standard for one-time initialization.

New CIR Operation: GuardedInitOp

• Defined in CIROps.td as cir.guarded.init
• Represents the guarded initialization pattern for static locals
• Contains an init_region for initialization code
• Attributes: thread_safe, perform_init
• Parameters: guard variable (pointer), static variable (symbol)

CIRGen Changes:

• CIRGenFunction: Added emitCXXGuardedInit() and emitCXXGlobalVarDeclInit()
• CIRGenCXXABI: Added virtual emitGuardedInit() method for ABI abstraction
• CIRGenItaniumCXXABI: Core implementation with guard variable creation
  • Follows Itanium C++ ABI specification
  • Guard variable mangling: _ZGVZ{function}E{variable}
  • Guard type: i64 (generic ABI) or i8 (internal linkage)
  • Zero-initialized with same linkage as guarded variable
• CIRGenDecl: Integration at two call sites
  • Dynamic initialization (constant emission fails)
  • Constant init with non-trivial destructor

Lowering Implementation:

• LoweringPrepare: Converts GuardedInitOp to control flow
• Pattern: if (!guard) { init_code; guard = 1; }
• Thread-safe guards marked as NYI (requires _cxa_guard* runtime calls)

Verifier:

• Validates guard variable is pointer type
• Ensures init_region has exactly one block
• Confirms termination with cir.yield

Follows ClangIR's deferred lowering principle:

• CIRGen emits high-level GuardedInitOp with initialization semantics
• LoweringPrepare handles platform-specific lowering
• Enables future optimizations (e.g., guard elision)

Copies CodeGen structure from:

• clang/lib/CodeGen/CGDecl.cpp:353-402 (AddInitializerToStaticVarDecl)
• clang/lib/CodeGen/CGDeclCXX.cpp:176-232 (EmitCXXGlobalVarDeclInit)
• clang/lib/CodeGen/ItaniumCXXABI.cpp:2691-2906 (EmitGuardedInit)

New Test:

• clang/test/CIR/CodeGen/static-var-init.cpp
  • Tests CIR emission and LLVM lowering
  • Covers: destructor-only, dynamic init, constant init cases
  • Validates guard variable creation and initialization pattern

Crash Tests Fixed:

• static-var-guarded-init.cpp - Static with non-trivial destructor
• static-var-dyn-cast.cpp - Static with template constructor
• Both updated with CHECK directives, XFAIL removed

✅ Static variables with non-trivial destructors
✅ Static variables with dynamic initialization
✅ Static variables with constant init + non-trivial destructor
✅ Reference static variables
⏳ Thread-safe guards (NYI, marked for future work)

Implements Section 3.3.2 "One-time construction API":

• Guard variable semantics (first byte = initialization status)
• Proper mangling for guard variables
• Destructor registration via existing __cxa_atexit mechanism

All code follows ClangIR standards:

• CIRGen: camelBack for members/parameters/variables
• Formatted with clang-format
• Includes comprehensive comments explaining design

Thread-safe guards (Phase 2):

• Implement __cxa_guard_acquire/release/abort calls
• Add atomic operations for guard variable access
• Handle exception cleanup during initialization

Potential optimizations:

• Guard elision when initialization proven safe
• Constant folding for always-initialized cases
• TLS optimization for thread-local statics