[coroutines] Experimenting with compile-time size check

[GorNishanov]

Summary

clang part of: https://reviews.llvm.org/D54500

! Not intended to be upstreamed into clang trunk at the moment!

include/clang/Basic/Builtins.def

 BUILTIN(__builtin_nontemporal_load, "v.", "t")
 
 // Coroutine intrinsics.
 BUILTIN(__builtin_coro_resume, "vv*", "")
 BUILTIN(__builtin_coro_destroy, "vv*", "")
 BUILTIN(__builtin_coro_done, "bv*", "n")
 BUILTIN(__builtin_coro_promise, "v*v*IiIb", "n")
 
+BUILTIN(__builtin_coro_size_chk, "vz", "n")
 BUILTIN(__builtin_coro_size, "z", "n")
 BUILTIN(__builtin_coro_frame, "v*", "n")
 BUILTIN(__builtin_coro_noop, "v*", "n")
 BUILTIN(__builtin_coro_free, "v*v*", "n")
 
 BUILTIN(__builtin_coro_id, "v*Iiv*v*v*", "n")
 BUILTIN(__builtin_coro_alloc, "b", "n")
 BUILTIN(__builtin_coro_begin, "v*v*", "n")

lib/CodeGen/CGBuiltin.cpp

if (llvm::GlobalVariable *GV =
       return RValue::get(llvm::ConstantExpr::getBitCast(GV, CGM.Int8PtrTy));
     break;
   }
 
   case Builtin::BI__fastfail:
     return RValue::get(EmitMSVCBuiltinExpr(MSVCIntrin::__fastfail, E));
 
   case Builtin::BI__builtin_coro_size: {
-    auto & Context = getContext();
+    auto &Context = getContext();
     auto SizeTy = Context.getSizeType();
     auto T = Builder.getIntNTy(Context.getTypeSize(SizeTy));
     Value *F = CGM.getIntrinsic(Intrinsic::coro_size, T);
     return RValue::get(Builder.CreateCall(F));
   }
+  case Builtin::BI__builtin_coro_size_chk: {
+    auto &Context = getContext();
+    auto SizeTy = Context.getSizeType();
+    auto T = Builder.getIntNTy(Context.getTypeSize(SizeTy));
+    Value *F = CGM.getIntrinsic(Intrinsic::coro_size_chk, T);
+    return RValue::get(Builder.CreateCall(F, EmitScalarExpr(E->getArg(0))));
+  }
 
   case Builtin::BI__builtin_coro_id:
     return EmitCoroutineIntrinsic(E, Intrinsic::coro_id);
   case Builtin::BI__builtin_coro_promise:
     return EmitCoroutineIntrinsic(E, Intrinsic::coro_promise);
   case Builtin::BI__builtin_coro_resume:
     return EmitCoroutineIntrinsic(E, Intrinsic::coro_resume);
   case Builtin::BI__builtin_coro_frame: