Implement extension GL_AMD_gpu_shader_int16

- Add int16 types (int16_t, uint16_t, i16vec, u16vec).
- Add int16 support to GLSL operators.
- Add int16 type conversions (to int16, from int16).
- Add int16 built-in functions.

gtests/Spv.FromFile.cpp

@@ -396,6 +396,7 @@ INSTANTIATE_TEST_CASE_P(
     Glsl, CompileVulkanToSpirvTestAMD,
     ::testing::ValuesIn(std::vector<std::string>({
         "spv.float16.frag",
+        "spv.int16.frag",
         "spv.shaderBallotAMD.comp",
         "spv.textureGatherBiasLod.frag"
     })),