[lldb][TypeSystemClang][NFC] Make TemplateParameterInfos members private

This patch makes the members of `TemplateParameterInfos` only accessible
via public APIs. The motivation for this is that
`TemplateParameterInfos` attempts to maintain two vectors in tandem
(`args` for the template arguments and `names` for the corresponding
name). Working with this structure as it's currently designed makes
it easy to run into out-of-bounds accesses later down the line.

This patch proposes to introduce a new
`TemplateParameterInfos::InsertArg` which is the only way to
set the `TemplateArgument` and name of an entry and since we
require both to be specified we maintain the vectors in sync
out-of-the-box.

To avoid adding non-const getters just for unit-tests a new
`TemplateParameterInfosManipulatorForTests` is introduced
that can be used to control internal state from tests.