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

gtests/Hlsl.FromFile.cpp

@@ -175,6 +175,7 @@ INSTANTIATE_TEST_CASE_P(
         {"hlsl.logical.binary.vec.frag", "main"},
         {"hlsl.logicalConvert.frag", "main"},
         {"hlsl.logical.unary.frag", "main"},
+        {"hlsl.loopattr.frag", "main"},
         {"hlsl.namespace.frag", "main"},
         {"hlsl.nonint-index.frag", "main"},
         {"hlsl.matNx1.frag", "main"},