[Clang][HLSL] Add environment parameter to Clang availability attribute

[hekota]

Add environment parameter to Clang availability attribute. The allowed values for this parameter are a subset of the llvm::Triple environment component valid values. If the environment parameters is present, the declared availability attribute applies to targets with the same platform and environment.

This parameter will be initially used for annotating HLSL functions for the shadermodel platform. In HLSL the built-in function availability can depend not just on the shader model version but also on the target shader stage. For example derivative functions ddx and ddy were introduced in Shader Model 2.0 for use only in pixel shaders, in Shader Model 6.6 their support was extended to compute, mesh and amplification shaders, and they are not available in any other shader stage.

With the new environment parameter the ddx availability annotation would look like this:

__attribute__((availability(shadermodel, introduced = 2.0, environment = pixel)))
__attribute__((availability(shadermodel, introduced = 6.6, environment = compute)))
__attribute__((availability(shadermodel, introduced = 6.6, environment = mesh)))
__attribute__((availability(shadermodel, introduced = 6.6, environment = amplification)))
__attribute__((clang_builtin_alias(__builtin_hlsl_ddx)))
float ddx(float val);

Related to microsoft/hlsl-specs#204.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Helena Kotas (hekota)

Add `environment` parameter to Clang availability attribute. The allowed values for this parameter are a subset of the `llvm::Triple` environment component valid values. If the `environment` parameters is present, the declared availability attribute applies to targets with the same platform and environment.

This parameter will be initially used for annotating HLSL functions for the shadermodel platform. In HLSL the built-in function availability can depend not just on the shader model version but also on the target shader stage. For example derivative functions ddx and ddy were introduced in Shader Model 2.0 for use only in pixel shaders, in Shader Model 6.6 their support was extended to compute, mesh and amplification shaders, and they are not available in any other shader stage.

With the new environment parameter the ddx availability annotation would look like this:

__attribute__((availability(shadermodel, introduced = 2.0, environment = pixel)))
__attribute__((availability(shadermodel, introduced = 6.6, environment = compute)))
__attribute__((availability(shadermodel, introduced = 6.6, environment = mesh)))
__attribute__((availability(shadermodel, introduced = 6.6, environment = amplification)))
__attribute__((clang_builtin_alias(__builtin_hlsl_ddx)))
float ddx(float val);

Related to microsoft/hlsl-specs#204.