Add test for construction of void.

This fails exactly as it should, but we had no test for it.

Change-Id: I0aa3307c444f2c9bc3512ff43b784a56a7c09856
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/472449
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>

gn/sksl_tests.gni

@@ -220,6 +220,7 @@ sksl_error_tests = [
   "/sksl/errors/VectorInlinedIndexOutOfRange.sksl",
   "/sksl/errors/VectorSlice.sksl",
   "/sksl/errors/VertexEarlyReturn.vert",
+  "/sksl/errors/VoidConstructor.sksl",
   "/sksl/errors/VoidInStruct.sksl",
   "/sksl/errors/VoidVariable.sksl",
   "/sksl/errors/WhileTypeMismatch.sksl",

resources/sksl/errors/VoidConstructor.sksl

@@ -0,0 +1,3 @@
+void main() {
+    void();
+}

tests/sksl/errors/VoidConstructor.glsl

@@ -0,0 +1,4 @@
+### Compilation failed:
+
+error: 2: cannot construct 'void'
+1 error