SPV: Address #989: Don't add Location to built-ins when automapping.

Test/spv.noBuiltInLoc.vert

@@ -0,0 +1,13 @@
+#version 450 core
+
+layout(location = 0)
+in  vec4 foo;
+
+layout(location = 0)
+out vec4 bar;
+
+void main()
+{
+    bar = foo;
+    gl_Position = foo;
+}