HLSL: implement [unroll] and [loop] attributes

This adds infrastructure suitable for any front end to create SPIR-V loop
control flags.  The only current front end doing so is HLSL.

[unroll] turns into spv::LoopControlUnrollMask
[loop] turns into spv::LoopControlDontUnrollMask
no specification means spv::LoopControlMaskNone

Test/hlsl.loopattr.frag

@@ -0,0 +1,14 @@
+
+float4 main() : SV_Target0
+{
+    // Unroll hint
+    [unroll(5) ] for (int x=0; x<5; ++x);
+
+    // Don't unroll hint
+    [loop] for (int y=0; y<5; ++y);
+
+    // No hint
+    for (int z=0; z<5; ++z);
+    
+    return 0;
+}