Fix SI-6578. Deprecated `askType` because of possible race conditions in...

... type checker.

AskType triggers type-checks the given source and returns a typed tree. If that
source is already loaded (a precondition), the background compilation loop may actually
be compiling that same source. The new type checker run may then get into an inconsistent
state and try to add twice the same synthetic members, like `canEqual`.

Most of the times, `askLoadedTyped` (that waits for the type checker to finish, and
returns the most recent typed tree) *is* the right way to go.

Removed occurrences of the deprecated method in tests and interactive.REPL.

@reviewby @huitseeker,@odersky