SPV: Don't add clip/cull distance capabilities unless used.

These capabalities were added on declaration of the members, but
that is considered too aggressive, as those members are automatically
declared in some shaders that don't use them.  Now, actual access
is needed to make the capabalities be declared.

Test/baseResults/spv.bool.vert.out

@@ -10,8 +10,6 @@ Linked vertex stage:
 // Id's are bound by 49
 
                               Capability Shader
-                              Capability ClipDistance
-                              Capability CullDistance
                1:             ExtInstImport  "GLSL.std.450"
                               MemoryModel Logical GLSL450
                               EntryPoint Vertex 4  "main" 24