[NFCI] More TypeCategoryImpl refactoring.

The main aim of this patch is to delete the remaining instances of code
reaching into the internals of `TypeCategoryImpl`. I made the following
changes:

- Add some more methods to `TieredFormatterContainer` and
  `TypeCategoryImpl` to expose functionality that is implemented in
  `FormattersContainer`.

- Add new overloads of `TypeCategoryImpl::AddTypeXXX` to make it easier
  to add formatters to categories without reaching into the internal
  `FormattersContainer` objects.

- Remove the `GetTypeXXXContainer` and `GetRegexTypeXXXContainer`
  accessors from `TypeCategoryImpl` and update all call sites to use the
  new methods instead.

Differential Revision: https://reviews.llvm.org/D135399