[lldb][NFC] Overload raw_ostream operator << for ConstString

Summary: We are not doing this very often, but sometimes it's convenient when I can just << ConstStrings into llvm::errs() during testing.

Reviewers: labath, JDevlieghere

Reviewed By: labath, JDevlieghere

Subscribers: JDevlieghere

Differential Revision: https://reviews.llvm.org/D80310