CPlusPlusLanguage: Use new demangler API to implement type substitution

Summary:
Now that llvm demangler supports more generic customization, we can
implement type substitution directly on top of this API. This will allow
us to remove the specialized hooks which were added to the demangler to
support this use case.

Reviewers: sgraenitz, erik.pilkington, JDevlieghere

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D54074

llvm-svn: 346233