[CIR][Test] Add divergence test for vtable constant issue

This test documents a divergence between CIR and CodeGen where CIR
incorrectly emits vtables as mutable globals instead of immutable constants.

**Issue:**
CIR emits vtables with 'global' linkage:
  @_ZTV4Base = linkonce_odr global { [3 x ptr] } ...

CodeGen correctly emits them as 'constant':
  @_ZTV4Base = linkonce_odr unnamed_addr constant { [3 x ptr] } ... comdat

**Root Cause:**
In CIRGenModule.cpp:4081, createOrReplaceCXXRuntimeVariable() calls
createGlobalOp() without passing isConstant=true parameter.

**Impact:**
1. Vtables placed in .data instead of .rodata section
2. Security vulnerability - writable vtables enable exploits
3. Missed optimization opportunities
4. Diverges from CodeGen without justification

**Additional Issues:**
CIR also misses 'unnamed_addr' and 'comdat' attributes that CodeGen emits.

The test is marked XFAIL until the fix is implemented.

Test Plan:
Build and verify test fails as expected (documenting divergence):
  bin/llvm-lit -v ../../clang/test/CIR/CodeGen/vtable-constant-divergence.cpp

Author: lanza

clang/test/CIR/CodeGen/vtable-constant-divergence.cpp

@@ -0,0 +1,31 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-llvm %s -o %t-cir.ll
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -emit-llvm %s -o %t-codegen.ll
+// RUN: diff %t-cir.ll %t-codegen.ll
+
+// XFAIL: *
+
+// This test documents a divergence between CIR and CodeGen:
+// CIR emits vtables as 'global' (mutable) instead of 'constant' (immutable).
+// This is a bug that needs to be fixed.
+//
+// Expected (CodeGen):
+//   @_ZTV4Base = linkonce_odr unnamed_addr constant { [3 x ptr] } ...
+//
+// Actual (CIR):
+//   @_ZTV4Base = linkonce_odr global { [3 x ptr] } ...
+//
+// The vtable should be marked as 'constant' because:
+// 1. Vtables are never modified at runtime
+// 2. They should be placed in .rodata (read-only data section)
+// 3. Writable vtables are a security vulnerability
+// 4. CodeGen has always emitted them as constant
+
+class Base {
+public:
+  virtual void foo() {}
+};
+
+void test() {
+  Base b;
+  b.foo();
+}