[linux-port] Add dxc -spirv test (#1409)

Due to existing bugs in debug builds, this couldn't be committed
before now. It should catch that and other issues by being tested
regularly. It just compiles a very simple HLSL shader to spirv and
compares with a previously generated spirv output file

.travis.yml

@@ -72,4 +72,6 @@ script:
   - ./bin/dxc -T ps_6_0 ../tools/clang/test/CodeGenSPIRV/passthru-ps.hlsl2spv
   - ./bin/dxc -T ps_6_0 -Fo passthru-ps.dxil ../tools/clang/test/CodeGenSPIRV/passthru-ps.hlsl2spv
   - cmp passthru-ps.dxil ../tools/clang/test/CodeGenSPIRV/passthru-ps.${DXC_BUILD_TYPE}.dxil
+  - ./bin/dxc -T ps_6_0 -Fo passthru-ps.spv ../tools/clang/test/CodeGenSPIRV/passthru-ps.hlsl2spv -spirv
+  - cmp passthru-ps.spv ../tools/clang/test/CodeGenSPIRV/passthru-ps.spv
   - ./bin/clang-spirv-tests --spirv-test-root ../tools/clang/test/CodeGenSPIRV/