[lldb] Change formatter helper function parameter list to remove ConstString

All of these functions take a ConstString for the type_name,
but this isn't really needed for two reasons:
1.) This parameter is always constructed from a static c-string
  constant.
2.) They are passed along to to `AddTypeSummary` as a StringRef anyway.

Differential Revision: https://reviews.llvm.org/D148050