Avoid Not a trait. diagnostic if a diagnostic was already reported.

[ilyalesokhin-starkware]

This change is 

[ilyalesokhin-starkware]

crates/cairo-lang-semantic/src/expr/test_data/generics line 556 at r1 (raw file):

 --> lib.cairo:4:19
impl MyImpl<T> of MyTrait<T, T> {
                  ^***********^

I think we should remove this error.

Code quote:

error: Not a trait.
 --> lib.cairo:4:19
impl MyImpl<T> of MyTrait<T, T> {
                  ^***********^

[orizi]

Reviewed 5 of 5 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ilyalesokhin-starkware)