Allow void macro result (#11286)

* allow void macro result
* add test for void macro result type

tests/macros/tmacro6.nim

@@ -0,0 +1,75 @@
+discard """
+errormsg: "expression '123' is of type 'int literal(123)' and has to be discarded"
+line: 71
+"""
+
+import macros
+
+proc foo(a,b,c: int): int =
+  result += a
+  result += b
+  result += c
+
+macro bar(a,b,c: int): int =
+  result = newCall(ident"echo")
+  result.add a
+  result.add b
+  result.add c
+
+macro baz(a,b,c: int): int =
+  let stmt = nnkStmtListExpr.newTree()
+  stmt.add newCall(ident"echo", a)
+  stmt.add newCall(ident"echo", b)
+  stmt.add newCall(ident"echo", c)
+  stmt.add newLit(123)
+  return c
+
+# test no result type with explicit return
+
+macro baz2(a,b,c: int) =
+  let stmt = nnkStmtListExpr.newTree()
+  stmt.add newCall(ident"echo", a)
+  stmt.add newCall(ident"echo", b)
+  stmt.add newCall(ident"echo", c)
+  return stmt
+
+# test explicit void type with explicit return
+
+macro baz3(a,b,c: int): void =
+  let stmt = nnkStmtListExpr.newTree()
+  stmt.add newCall(ident"echo", a)
+  stmt.add newCall(ident"echo", b)
+  stmt.add newCall(ident"echo", c)
+  return stmt
+
+# test no result type with result variable
+
+macro baz4(a,b,c: int) =
+  result = nnkStmtListExpr.newTree()
+  result.add newCall(ident"echo", a)
+  result.add newCall(ident"echo", b)
+  result.add newCall(ident"echo", c)
+
+# test explicit void type with result variable
+
+macro baz5(a,b,c: int): void =
+  let result = nnkStmtListExpr.newTree()
+  result.add newCall(ident"echo", a)
+  result.add newCall(ident"echo", b)
+  result.add newCall(ident"echo", c)
+
+macro foobar1(): int =
+  result = quote do:
+    echo "Hello World"
+    1337
+
+echo foobar1()
+
+# this should create an error message, because 123 has to be discarded.
+
+macro foobar2() =
+  result = quote do:
+    echo "Hello World"
+    123
+
+echo foobar2()