fix(experimental elaborator): Only add_generics once for each item

[jfecher]

Description

Problem*

Resolves an issue introduced by #5082 where add_generics would be called multiple times on a given item. Once when declaring the FuncMeta beforehand, and again when defining it afterward. This is an issue because add_generics will give a fresh set of generics each time. This meant the generics in a function's signature weren't the same type variables as the ones used internally, which led to type errors.

Summary*

Additional Context

Down to 400 errors in the stdlib down from 1050

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[jfecher]

Wrong merge target