Emit warning when using the Yul optimiser

test/cmdlineTests/gas_test_abiv2_optimize_yul/err

@@ -1,3 +1,4 @@
+Warning: The Yul optimiser is still experimental. Do not use it in production unless correctness of generated code is verified with extensive tests.
 gas_test_abiv2_optimize_yul/input.sol:2:1: Warning: Experimental features are turned on. Do not use experimental features on live deployments.
 pragma experimental ABIEncoderV2;
 ^-------------------------------^

test/cmdlineTests/standard_optimizer_yul/input.json

@@ -0,0 +1,17 @@
+{
+	"language": "Solidity",
+	"sources":
+	{
+		"A":
+		{
+			"content": "pragma solidity >=0.0; contract C { function f() public pure {} }"
+		}
+	},
+	"settings":
+	{
+		"optimizer": {
+			"enabled": true,
+			"details": { "yul": true }
+		}
+	}
+}

test/cmdlineTests/standard_optimizer_yul/output.json

@@ -0,0 +1 @@
+{"errors":[{"component":"general","formattedMessage":"Warning: The Yul optimiser is still experimental. Do not use it in production unless correctness of generated code is verified with extensive tests.\n","message":"The Yul optimiser is still experimental. Do not use it in production unless correctness of generated code is verified with extensive tests.","severity":"warning","type":"Warning"}],"sources":{"A":{"id":0}}}

test/cmdlineTests/standard_optimizer_yulDetails/input.json

@@ -0,0 +1,16 @@
+{
+	"language": "Solidity",
+	"sources":
+	{
+		"A":
+		{
+			"content": "pragma solidity >=0.0; contract C { function f() public pure {} }"
+		}
+	},
+	"settings":
+	{
+		"optimizer": {
+			"details": { "yul": true, "yulDetails": {} }
+		}
+	}
+}

test/cmdlineTests/standard_optimizer_yulDetails/output.json

@@ -0,0 +1 @@
+{"errors":[{"component":"general","formattedMessage":"Warning: The Yul optimiser is still experimental. Do not use it in production unless correctness of generated code is verified with extensive tests.\n","message":"The Yul optimiser is still experimental. Do not use it in production unless correctness of generated code is verified with extensive tests.","severity":"warning","type":"Warning"}],"sources":{"A":{"id":0}}}