[NF] Port inStream changes from #2319.

- Port the changes made to inStream in the old frontend to the new,
  in particular setting the Global.isInStream global root to avoid
  the backend failing when trying to access it.

Belonging to [master]:
  - #2329
  - OpenModelica/OpenModelica-testsuite#907

Compiler/NFFrontEnd/NFBuiltinFuncs.mo

@@ -214,6 +214,10 @@ constant Function MAX_REAL = Function.FUNCTION(Path.IDENT("max"),
   InstNode.EMPTY_NODE(), {REAL_PARAM, REAL_PARAM}, {REAL_PARAM}, {}, {},
     Type.REAL(), DAE.FUNCTION_ATTRIBUTES_BUILTIN, Pointer.createImmutable(true));
 
+constant Function POSITIVE_MAX_REAL = Function.FUNCTION(Path.IDENT("$OMC$PositiveMax"),
+  InstNode.EMPTY_NODE(), {REAL_PARAM, REAL_PARAM}, {REAL_PARAM}, {}, {},
+    Type.REAL(), DAE.FUNCTION_ATTRIBUTES_BUILTIN, Pointer.createImmutable(true));
+
 constant Function IN_STREAM = Function.FUNCTION(Path.IDENT("inStream"),
   InstNode.EMPTY_NODE(), {REAL_PARAM}, {REAL_PARAM}, {}, {},
     Type.REAL(), DAE.FUNCTION_ATTRIBUTES_BUILTIN, Pointer.createImmutable(true));

Compiler/NFFrontEnd/NFConnectEquations.mo

@@ -60,6 +60,7 @@ import NFInstNode.InstNode;
 import NFClass.Class;
 import Binding = NFBinding;
 import NFFunction.Function;
+import Global;
 
 constant Expression EQ_ASSERT_STR =
   Expression.STRING("Connected constants/parameters must be equal");
@@ -78,6 +79,8 @@ protected
   potFunc potfunc;
   Expression flowThreshold;
 algorithm
+  setGlobalRoot(Global.isInStream, NONE());
+
   //potfunc := if Config.orderConnections() then
   //  generatePotentialEquationsOrdered else generatePotentialEquations;
   potfunc := generatePotentialEquations;
@@ -528,8 +531,10 @@ algorithm
     flow_threshold := flowThreshold;
   end if;
 
-  positiveMaxCall := Expression.CALL(Call.makeBuiltinCall(NFBuiltinFuncs.MAX_REAL,
+  positiveMaxCall := Expression.CALL(Call.makeBuiltinCall(NFBuiltinFuncs.POSITIVE_MAX_REAL,
     {flowExp, flow_threshold}));
+
+  setGlobalRoot(Global.isInStream, SOME(true));
 end makePositiveMaxCall;
 
 function evaluateOperatorExp