[WebAssembly] Restore __builtin_wasm_rethrow builtin

[aheejin]

Summary

Restore the __builtin_wasm_rethrow builtin deleted in D37931. On second
thought, it appears it can be used to implement __cxa_rethrow.

[dschuff]

Or you could just call it a revert of the previous patch, if you wanted.

[dschuff]

(btw if there's a link in the commit message to the review, it should close automatically after commit)

[aheejin]

I didn't close it explicitly. I just committed the change. I don't know why my CLs do not say "closed by commit rxxxx" like other CLs...

include/clang/Basic/BuiltinsWebAssembly.def

 
 // Note that current_memory is not "c" (readnone) because it must be sequenced
 // with respect to grow_memory calls.
 BUILTIN(__builtin_wasm_current_memory, "z", "n")
 BUILTIN(__builtin_wasm_grow_memory, "zz", "n")
 
 // Exception handling builtins.
 BUILTIN(__builtin_wasm_throw, "vUiv*", "r")
+BUILTIN(__builtin_wasm_rethrow, "v", "r")
 
 #undef BUILTIN

lib/CodeGen/CGBuiltin.cpp

case WebAssembly::BI__builtin_wasm_grow_memory: {
     return Builder.CreateCall(Callee, X);
   }
   case WebAssembly::BI__builtin_wasm_throw: {
     Value *Tag = EmitScalarExpr(E->getArg(0));
     Value *Obj = EmitScalarExpr(E->getArg(1));
     Value *Callee = CGM.getIntrinsic(Intrinsic::wasm_throw);
     return Builder.CreateCall(Callee, {Tag, Obj});
   }
+  case WebAssembly::BI__builtin_wasm_rethrow: {
+    Value *Callee = CGM.getIntrinsic(Intrinsic::wasm_rethrow);
+    return Builder.CreateCall(Callee);
+  }
 
   default:
     return nullptr;
   }
 }

test/CodeGen/builtins-wasm.c

 // WEBASSEMBLY64: call i64 @llvm.wasm.grow.memory.i64(i64 %{{.*}})
 }
 
 void f3(unsigned int tag, void *obj) {
   return __builtin_wasm_throw(tag, obj);
 // WEBASSEMBLY32: call void @llvm.wasm.throw(i32 %{{.*}}, i8* %{{.*}})
 // WEBASSEMBLY64: call void @llvm.wasm.throw(i32 %{{.*}}, i8* %{{.*}})
 }
+
+void f4() {
+  return __builtin_wasm_rethrow();
+// WEBASSEMBLY32: call void @llvm.wasm.rethrow()
+// WEBASSEMBLY64: call void @llvm.wasm.rethrow()
+}