Add test for variables of type void.

We should, of course, detect this and report an error.

Change-Id: I42b3be6e714a1f367d3251842506a384f2afe019
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/472447
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>

gn/sksl_tests.gni

@@ -221,6 +221,7 @@ sksl_error_tests = [
   "/sksl/errors/VectorInlinedIndexOutOfRange.sksl",
   "/sksl/errors/VectorSlice.sksl",
   "/sksl/errors/VertexEarlyReturn.vert",
+  "/sksl/errors/VoidVariable.sksl",
   "/sksl/errors/WhileTypeMismatch.sksl",
 ]
 

resources/sksl/errors/VoidVariable.sksl

@@ -0,0 +1,5 @@
+// Expect 2 errors
+void func();
+
+void a;
+void b = func();

tests/sksl/errors/VoidVariable.glsl

@@ -0,0 +1,4 @@
+### Compilation failed:
+
+error: 5: opaque type 'void' cannot use initializer expressions
+1 error