Code with unary plus is recognized as l-value

The following code with a (useless) unary plus is validated by glslangvalidator while it should raise an error for the assignment to a non-lvalue.

It correctly raise an error for the unary minus.

```
#version 320 es

void main() {
int var1 = 5;
int var2 = + var1 += 3;
}
```

Start by reproducing the example with glslangvalidator and compare its handling of unary plus with the unary-minus case described in the issue. The work is done when the unary-plus assignment to a non-lvalue is rejected as invalid.

Written by the indexing model from the issue text.