[NFCI] Simplify TypeCategoryImpl for-each callbacks.

The callback system to iterate over every formatter of a given kind in
a TypeCategoryImpl is only used in one place (the implementation of
`type {formatter_kind} list`), and it's too convoluted for the sake of
unused flexibility.

This change changes it so that only one callback is passed to `ForEach`
(instead of a callback for exact matches and another one for regex
matches), and moves the iteration logic to `TieredFormatterContainer`
to avoid duplication.

If in the future we need different logic in the callback depending on
exact/regex match, the callback can get the type of formatter matching
used from the TypeMatcher argument anyway.

Differential Revision: https://reviews.llvm.org/D134771