Roll external/spirv-headers/ 8b911bd2b..de99d4d83 (8 commits) (#2682)

https://github.com/KhronosGroup/SPIRV-Headers/compare/8b911bd2ba37...de99d4d834ae

$ git log 8b911bd2b..de99d4d83 --date=short --no-merges --format='%ad %ae %s'
2019-06-12 dneto Add Volatile to Memory Semantics, for SPV_KHR_vulkan_memory_model
2019-06-10 ehsannas Add grammar and symbols for UserTypeGOOGLE extension to unified1.
2019-06-07 cepheus Add missing "version" : "None" to a bunch of reserved enumerants.
2019-06-07 cepheus Add more detail about reserving tokens to the README.
2019-06-07 cepheus Restore numerical order in enumerants.
2019-05-18 mchiasson Update CMakeLists.txt
2019-05-16 mchiasson updated as per code review
2019-05-11 mchiasson cmake development configuration package Fixes #104

Created with:
  roll-dep external/spirv-headers