Disallow assignment to expressions
Disallow assignment to expressions

Fixes #6824

(cherry picked from commits 0b077162a32d37b81c302ab26523efda05de3913
and dcc4ee21c14c1d3fc40c2058e41f853e8ce989c6)

pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1,pixelpicosean/my-godot-2.1