Use LLVM's new ItaniumPartialDemangler in LLDB

Summary:
Replace the existing combination of FastDemangle and the fallback to llvm::itaniumDemangle() with LLVM's new ItaniumPartialDemangler. It slightly reduces complexity and slightly improves performance, but doesn't introduce conceptual changes. This patch is preparing for more fundamental improvements on LLDB's demangling approach.

Reviewers: friss, jingham, erik.pilkington, labath, clayborg, mgorny, davide, JDevlieghere

Reviewed By: JDevlieghere

Subscribers: teemperor, JDevlieghere, labath, clayborg, davide, lldb-commits, mgorny, erik.pilkington

Differential Revision: https://reviews.llvm.org/D49612

llvm-svn: 337931