Fix SPIR-V for HLSL EvaluateAttribute* of interpolants in structs

Generate load of interpolant for first operand to GLSLstd450
InterpolateAt* SPIR-V ops. This allows the interpolants to
propagate from the input struct in the wrapper around main
into the shader during HLSL legalization. A new pass has been
added to legalization which will remove the load and replace
with the pointer of the load to create valid external
interpolate op.

Fixes #2584

known_good.json

@@ -5,14 +5,14 @@
       "site" : "github",
       "subrepo" : "KhronosGroup/SPIRV-Tools",
       "subdir" : "External/spirv-tools",
-      "commit" : "c79edd260c2b503f0eca57310057b4a100999cc5"
+      "commit" : "48007a5c7f7cc671b391bebd46e87fd6edc6c24b"
     },
     {
       "name" : "spirv-tools/external/spirv-headers",
       "site" : "github",
       "subrepo" : "KhronosGroup/SPIRV-Headers",
       "subdir" : "External/spirv-tools/external/spirv-headers",
-      "commit" : "75b30a659c8a4979104986652c54cc421fc51129"
+      "commit" : "f88a1f98fa7a44ccfcf33d810c72b200e7d9a78a"
     }
   ]
 }