[clang][CodeGen] Don't crash on sizeless output.

This fixes issue #63878 caused by creating an integer with zero
bitwidth.

Author: Yeting Kuo

clang/lib/CodeGen/CGStmt.cpp

@@ -2751,7 +2751,10 @@ void CodeGenFunction::EmitAsmStmt(const AsmStmt &S) {
 
       if (RequiresCast) {
         unsigned Size = getContext().getTypeSize(QTy);
-        Ty = llvm::IntegerType::get(getLLVMContext(), Size);
+        if (Size)
+          Ty = llvm::IntegerType::get(getLLVMContext(), Size);
+        else
+          CGM.Error(S.getAsmLoc(), "Output operand is sizeless!");
       }
       ResultRegTypes.push_back(Ty);
       // If this output is tied to an input, and if the input is larger, then

clang/test/CodeGen/inline-asm-sizeless.c

@@ -0,0 +1,7 @@
+// RUN: not %clang_cc1 -S %s -o /dev/null 2>&1 | FileCheck %s
+
+// CHECK: error: Output operand is sizeless!
+void foo(void) {
+    extern long bar[];
+    asm ("" : "=r"(bar));
+}