SPV: Support test #pragma for generating the StorageBuffer storage class.

Longer term, this storage class should be generated based on the mode
of compilation.

Test/spv.storageBuffer.vert

@@ -0,0 +1,16 @@
+#version 450
+
+#pragma use_storage_buffer
+
+uniform ub {
+    vec4 a;
+} ubi;
+
+buffer bb {
+    vec4 b;
+} bbi;
+
+void main()
+{
+    gl_Position = ubi.a + bbi.b;
+}