[Sol - Yul] Add support for built-in gasleft().

libsolidity/codegen/ir/IRGeneratorForStatements.cpp

@@ -771,6 +771,11 @@ void IRGeneratorForStatements::endVisit(FunctionCall const& _functionCall)
 	{
 		break;
 	}
+	case FunctionType::Kind::GasLeft:
+	{
+		define(_functionCall) << "gas()\n";
+		break;
+	}
 	default:
 		solUnimplemented("FunctionKind " + toString(static_cast<int>(functionType->kind())) + " not yet implemented");
 	}

test/libsolidity/semanticTests/various/gasleft_decrease.sol

@@ -15,6 +15,8 @@ contract C {
     }
 }
 
+// ====
+// compileViaYul: also
 // ----
 // f() -> true
 // g() -> true