Lock semchecked ast for macros (#11883) [bugfix]

* reject to modify type checked AST
* add flag to back out
* Introduce legacy feature set.

tests/macros/tlocktypednode1.nim

@@ -0,0 +1,12 @@
+discard """
+errormsg: "typechecked nodes may not be modified"
+"""
+
+import macros
+
+macro doSomething(arg: typed): untyped =
+  echo arg.treeREpr
+  result = arg
+  result.add newCall(bindSym"echo", newLit(1))
+
+doSomething((echo(1); echo(2)))

tests/macros/tlocktypednode2.nim

@@ -0,0 +1,12 @@
+discard """
+errormsg: "typechecked nodes may not be modified"
+"""
+
+import macros
+
+macro doSomething(arg: typed): untyped =
+  echo arg.treeREpr
+  result = arg
+  result[0] = newCall(bindSym"echo", newLit(1))
+
+doSomething((echo(1); echo(2)))

tests/macros/tlocktypednode3.nim

@@ -0,0 +1,15 @@
+discard """
+errormsg: "typechecked nodes may not be modified"
+"""
+
+import macros
+
+macro doSomething(arg: typed): untyped =
+  echo arg.treeREpr
+  result = arg
+  result.add(
+    newCall(bindSym"echo", newLit(3)),
+    newCall(bindSym"echo", newLit(1))
+  )
+
+doSomething((echo(1); echo(2)))

tests/macros/tmacro7.nim

@@ -1,19 +1,20 @@
 discard """
-  output: '''calling!stuff
+output: '''
+calling!stuff
 calling!stuff
 '''
-  joinable: false
+disabled: true
 """
 
+# this test modifies an already semchecked ast (bad things happen)
+# this test relies on the bug #4547
 # issue #7792
 
 import macros
 
-
 proc callProc(str: string) =
   echo "calling!" & str
 
-
 macro testMacro(code: typed): untyped =
   let stmtList = newNimNode(nnkStmtList)
 
@@ -27,11 +28,9 @@ macro testMacro(code: typed): untyped =
 
   result = newEmptyNode()
 
-
 proc main() {.testMacro.} =
   echo "test"
   echo "test2"
 
-
 when isMainModule:
   main()