SPIR-V: support SV_InnerCoverage translation to SPIR-V FullyCoveredEXT..

[oscarbg]

Hi,
DX12 and DX11.3 support optional conservative rasterization.. there are tiers of support and tier3 adds HLSL builtin SV_InnerCoverage..
now that there is a Vulkan extension (VK_EXT_conservative_rasterization) for that and SPIR-V ext SPV_EXT_fragment_fully_covered that adds equivalent new SPIR-V FullyCoveredEXT built in would be nice to have in support for it..
right now only testable in HW by NV Titan V owners..
similar request to glslang devs:
KhronosGroup/glslang#1220

[antiagainst]

We'll need SPIRV-Tools disassembling functionality to test the CodeGen. Currently SPIRV-Tools does not recognize the FullyCoveredEXT bultin or the FragmentFullyCoveredEXT capability, since they are introduced in the unified1 SPIRV-Headers directory. After SPIRV-Tools migrates to use unified1 headers (KhronosGroup/SPIRV-Tools#1195), we can add support for it.