SPV: Give error on not assigning locations to I/O.

Also, provides an option to auto-assign locations.
Existing tests use this option, to avoid the error message,
however, it is not fully implemented yet.

Test/spv.noLocation.vert

@@ -0,0 +1,14 @@
+#version 450
+
+layout(location = 1) in vec4 in1;
+in vec4 in2;
+layout(location = 3) in vec4 in3;
+
+layout(location = 1) out vec4 out1;
+out vec4 out2;
+layout(location = 3) out vec4 out3;
+
+
+void main()
+{
+}