[clang] Fix crash related to _BitInt constant split (#112218)

9ad72df added split of _BitInt
constants when required. Before folding back, check that the constant
exists.

Author: Fznamznon

clang/lib/CodeGen/CGDecl.cpp

@@ -1945,7 +1945,7 @@ void CodeGenFunction::EmitAutoVarInit(const AutoVarEmission &emission) {
                                   replaceUndef(CGM, isPattern, constant));
     }
 
-    if (D.getType()->isBitIntType() &&
+    if (constant && D.getType()->isBitIntType() &&
         CGM.getTypes().typeRequiresSplitIntoByteArray(D.getType())) {
       // Constants for long _BitInt types are split into individual bytes.
       // Try to fold these back into an integer constant so it can be stored

clang/test/CodeGenCXX/ext-int.cpp

@@ -614,3 +614,18 @@ void TBAATest(_BitInt(sizeof(int) * 8) ExtInt,
 // NewStructPathTBAA-DAG: ![[EXTINT_TBAA_ROOT]] = !{![[CHAR_TBAA_ROOT]], i64 4, !"_BitInt(32)"}
 // NewStructPathTBAA-DAG: ![[EXTINT6_TBAA]] = !{![[EXTINT6_TBAA_ROOT:.+]], ![[EXTINT6_TBAA_ROOT]], i64 0, i64 1}
 // NewStructPathTBAA-DAG: ![[EXTINT6_TBAA_ROOT]] = !{![[CHAR_TBAA_ROOT]], i64 1, !"_BitInt(6)"}
+
+namespace A {
+template <int N> struct S {
+  using T = _BitInt(N);
+  T Data;
+};
+template <int N> void foo(S<N> B) {
+  const auto Var = B.Data;
+}
+
+void bar() {
+  S<2080> a;
+  foo(a);
+}
+}