Extension of cl_khr_icd to 2.0.0 with loader managed dispatch.

[Kerilk]

This is a formal description of the proposal discussed here:
#1003

[Kerilk]

@keryell was concerned clPlatformID pointer for a non ICD 2.0.0 compatible could have the same value as the tag.

One way to avoid this problem would be to have a second entry in the dispatch table set to the tag value (clUnloadCompiler comes to mind) and check that both of these have the tag while loading the library. If the tag were not present in both, the implementation would not be loaded.

[keryell]

@keryell was concerned clPlatformID pointer for a non ICD 2.0.0 compatible could have the same value as the tag.

One way to avoid this problem would be to have a second entry in the dispatch table set to the tag value (clUnloadCompiler comes to mind) and check that both of these have the tag while loading the library. If the tag were not present in both, the implementation would not be loaded.

Thanks. This is quite simpler and more portable than the solution I was thinking to.