Used for storing either AST generics or their lowered HIR version. Firstly we obtain
AST generics either from local function from AST index or from external function
through tcx. Next, at some point of generics processing we need to lower those
generics to HIR, for this purpose we use into_hir_generics that lowers AST generics
and replaces Ast variant with Hir. Such approach is useful as we can call this method
at any time knowing that lowering will occur at most only once. Then, in order to obtain generic
params or args we use hir_generics_or_empty or into_generic_args functions.
There also may be situations when we obtained AST generics but never lowered them to HIR,
meaning we did not propagate them and thus we do not need to generate generic params
(i.e., method call scenarios), in such a case this approach helps
a lot as if into_hir_generics will not be called then lowering will not happen.