SPV: Further refine OpenGL vs. Vulkan differences for SPIR-V.

Includes adding test cases to verify the differences.

Test/glspv.frag

@@ -0,0 +1,14 @@
+#version 330
+
+#ifdef GL_SPIRV
+#error GL_SPIRV is set ( correct, not an error )
+#if GL_SPIRV == 100
+#error GL_SPIR is 100
+#endif
+#endif
+
+void main()
+{
+}
+
+layout(input_attachment_index = 1) uniform subpassInput sub; // ERROR, no inputs