[CIR] Add crash test cases for const_cast l-value and cleanup null fixups

This commit adds two minimal test cases to clang/test/CIR/crashes/ for
known NYI (not yet implemented) features that cause crashes:

1. const-cast-lvalue.cpp - Tests CXXConstCastExpr l-value emission
   - Triggers assertion at CIRGenExpr.cpp:2764
   - Error: 'Use emitCastLValue below, remove me when adding testcase'
   - Minimal reproducer showing const_cast in struct member functions

2. cleanup-892-null-fixups.cpp - Tests null fixup popping in cleanups
   - Triggers UNREACHABLE at CIRGenCleanup.cpp:892
   - Missing implementation of EHScopeStack::popNullFixups()
   - Minimal reproducer with destructors and early returns

Both test cases were reduced from multi-megabyte preprocessed files
(from LLVM self-host build failures) down to <30 lines using creduce.

These tests are marked XFAIL and serve as regression tests for when
these features are implemented.

Test Plan:
- Verified both tests trigger their respective crashes with -fclangir
- Tests follow existing pattern in clang/test/CIR/crashes/
- Both tests are properly formatted and minimal

Author: lanza

clang/test/CIR/crashes/cleanup-892-null-fixups.cpp

@@ -0,0 +1,31 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir %s -o %t.cir
+// XFAIL: *
+//
+// Test for UNREACHABLE at CIRGenCleanup.cpp:892
+// Null fixups popping not yet implemented
+//
+// This test triggers the error:
+// "UNREACHABLE executed at CIRGenCleanup.cpp:892!"
+//
+// Original failure: cleanup_892 from LLVM build
+// Reduced from /tmp/Regex-8cd677.cpp
+
+inline namespace a {
+class c {
+public:
+  template <typename b> c(b);
+  ~c();
+};
+} // namespace a
+class d {
+  c e() const;
+};
+class aj {
+public:
+  ~aj();
+} an;
+c d::e() const {
+  aj ao;
+  return an;
+  c(0);
+}

clang/test/CIR/crashes/const-cast-lvalue.cpp

@@ -0,0 +1,28 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir %s -o %t.cir
+// XFAIL: *
+//
+// Test for CXXConstCastExpr l-value emission not implemented
+// Assertion at CIRGenExpr.cpp:2764
+//
+// This test triggers the assertion:
+// "Use emitCastLValue below, remove me when adding testcase"
+//
+// Original failure: assertion_emitcastlvalue from LLVM build
+// Reduced from /tmp/DeltaTree-af9b43.cpp
+
+int a;
+struct b {
+  using c = int;
+  static c d() { const_cast<int &>(a); }
+};
+struct e {
+  static bool f() { b::d; }
+};
+struct g {
+  static bool h() { e::f; }
+};
+struct j : g {
+  using i = int;
+  static i k() { h; }
+};
+void l() { j::k; }