[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