[TCGC] __raw of SdkCredentialType is the namespace instead of the decorator itself

[ArcturusZhang]

As we all know, SdkCredentialType corresponds to the @useAuth decorator, but its __raw is the namespace with that decorator instead of the decorator itself.
This would lead to when we want to report something wrong for the auth, the diagnostic will show up on the namespace instead of the auth itself.

This would be so confusing because in a lot of cases, the target of a decorator could be in a different file from the decorator itself.

For instance, if we have:

@useAuth(ApiKeyAuth<ApiKeyLocation.cookie, "my-cookie">)
namespace Service;

We should have the raw corresponding to the ApiKeyAuth<ApiKeyLocation.cookie, "my-cookie"> part, or at least the decorator itself.

[tadelesh]

in EmitContext, we could not get the decorator node directly, but only the info decorator helper exposed. idk if there is any convenient way to locate the original decorator node from the type. for your example, we get authentication info from getAuthentication helper and the result does not contain the original decorator info.