Remove std::function churn and wasted arena interning in MLIRGen (#203)

- Rewrite the three StringSwitch<std::function> utility-type tables
  (getEmbeddedTypeWithParam*/ManyParams) as enum dispatch; the old form
  constructed every case closure eagerly on each call
- Replace all std::bind sites with lambdas or member-function pointers
- Add concatFullNamespaceName() (no interning) and use it where the
  full name is transient (getNameOfFunction, enum type refs, dynamic
  import names); getFullNamespaceName keeps interning for stored names

Review doc item A4.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: ASDAlexander77