[PAC] Fix a crash when signing a pointer to a function with an incomplete enum parameter (#99595)

Use int as the underlying type when the enum type is incomplete.

Author: ahatanak

clang/lib/AST/ASTContext.cpp

@@ -3223,14 +3223,16 @@ static void encodeTypeForFunctionPointerAuth(const ASTContext &Ctx,
     OS << "<objc_object>";
     return;
 
-  case Type::Enum:
+  case Type::Enum: {
     // C11 6.7.2.2p4:
     //   Each enumerated type shall be compatible with char, a signed integer
     //   type, or an unsigned integer type.
     //
     // So we have to treat enum types as integers.
+    QualType UnderlyingType = cast<EnumType>(T)->getDecl()->getIntegerType();
     return encodeTypeForFunctionPointerAuth(
-        Ctx, OS, cast<EnumType>(T)->getDecl()->getIntegerType());
+        Ctx, OS, UnderlyingType.isNull() ? Ctx.IntTy : UnderlyingType);
+  }
 
   case Type::FunctionNoProto:
   case Type::FunctionProto: {

clang/test/CodeGen/ptrauth-function-type-discriminator.c

@@ -30,6 +30,13 @@ void (*test_constant_null)(int) = 0;
 // CHECK: @test_constant_cast = global ptr ptrauth (ptr @f, i32 0, i64 2712)
 void (*test_constant_cast)(int) = (void (*)(int))f;
 
+#ifndef __cplusplus
+// CHECKC: @enum_func_ptr = global ptr ptrauth (ptr @enum_func, i32 0, i64 2712)
+enum Enum0;
+void enum_func(enum Enum0);
+void (*enum_func_ptr)(enum Enum0) = enum_func;
+#endif
+
 // CHECK: @test_opaque = global ptr ptrauth (ptr @f, i32 0)
 void *test_opaque =
 #ifdef __cplusplus