update spirv-headers to pick up demote_to_helper_invocation

SPIRV/doc.cpp

@@ -958,6 +958,8 @@ const char* CapabilityString(int info)
     case CapabilityFragmentShaderPixelInterlockEXT:         return "CapabilityFragmentShaderPixelInterlockEXT";
     case CapabilityFragmentShaderShadingRateInterlockEXT:   return "CapabilityFragmentShaderShadingRateInterlockEXT";
 
+    case CapabilityDemoteToHelperInvocationEXT:             return "DemoteToHelperInvocationEXT";
+
     default: return "Bad";
     }
 }

Test/baseResults/spv.conditionalDemote.frag.out

 spv.conditionalDemote.frag
-Validation failed
 // Module Version 10000
 // Generated by (magic number): 80007
 // Id's are bound by 38
 
                               Capability Shader
-                              Capability Bad
+                              Capability DemoteToHelperInvocationEXT
                               Extension  "SPV_EXT_demote_to_helper_invocation"
                1:             ExtInstImport  "GLSL.std.450"
                               MemoryModel Logical GLSL450

known_good.json

@@ -12,7 +12,7 @@
       "site" : "github",
       "subrepo" : "KhronosGroup/SPIRV-Headers",
       "subdir" : "External/spirv-tools/external/spirv-headers",
-      "commit" : "8b911bd2ba37677037b38c9bd286c7c05701bcda"
+      "commit" : "123dc278f204f8e833e1a88d31c46d0edf81d4b2"
     }
   ]
 }