Add UAV (image) binding offset and HLSL register support

This PR adds:

1. The "u" register class for RW* objects.

2. --shift-image-bindings (== --sib), analogous to --shift-texture-bindings etc.

3. Case insensitive reg classes.

4. Tests for above.

gtests/Hlsl.FromFile.cpp

@@ -159,6 +159,7 @@ INSTANTIATE_TEST_CASE_P(
         {"hlsl.promotions.frag", "main"},
         {"hlsl.rw.atomics.frag", "main"},
         {"hlsl.rw.bracket.frag", "main"},
+        {"hlsl.rw.register.frag", "main"},
         {"hlsl.rw.scalar.bracket.frag", "main"},
         {"hlsl.rw.vec2.bracket.frag", "main"},
         {"hlsl.sample.array.dx10.frag", "main"},