Fix a crash on valid consteval code.

Not all constants are emitted within the context of a function, so use
the module's ASTContext instead because 1) that's the same as the
current function ASTContext, and 2) the module can never be null.

Fixes PR50787.

clang/lib/CodeGen/CGExprConstant.cpp

@@ -1369,7 +1369,7 @@ llvm::Constant *ConstantEmitter::tryEmitConstantExpr(const ConstantExpr *CE) {
   const Expr *Inner = CE->getSubExpr()->IgnoreImplicit();
   QualType RetType;
   if (auto *Call = dyn_cast<CallExpr>(Inner))
-    RetType = Call->getCallReturnType(CGF->getContext());
+    RetType = Call->getCallReturnType(CGM.getContext());
   else if (auto *Ctor = dyn_cast<CXXConstructExpr>(Inner))
     RetType = Ctor->getType();
   llvm::Constant *Res =

clang/test/CodeGenCXX/cxx20-consteval-crash.cpp

@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -std=c++20 %s -emit-llvm -o - | FileCheck %s
+
+namespace PR50787 {
+// This code would previously cause a crash.
+extern int x_;
+consteval auto& X() { return x_; }
+constexpr auto& x1 = X();
+auto x2 = X();
+
+// CHECK: @_ZN7PR507872x_E = external global i32, align 4
+// CHECK-NEXT: @_ZN7PR507872x1E = constant i32* @_ZN7PR507872x_E, align 8
+// CHECK-NEXT: @_ZN7PR507872x2E = global i32* @_ZN7PR507872x_E, align 4
+}
+