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/baseResults/hlsl.whileLoop.frag.out

@@ -171,7 +171,7 @@ gl_FragCoord origin is upper left
               28:             Label
                               Branch 32
               32:             Label
-                              LoopMerge 34 35 None
+                              LoopMerge 34 35 Unroll 
                               Branch 36
               36:             Label
                               BranchConditional 31 33 34