[lldb] Remove RTTI in ClangExternalASTSourceCommon based on a global map of known instances

Summary:
Currently we do our RTTI check for ClangExternalASTSourceCommon by using this global map of
ClangExternalASTSourceCommon where every instance is registering and deregistering itself
on creation/destruction. Then we can do the RTTI check by looking up in this map from ClangASTContext.

This patch removes this whole thing and just adds LLVM-style RTTI support to ClangExternalASTSourceCommon
which is possible with D71397.

Reviewers: labath, aprantl

Reviewed By: labath

Subscribers: JDevlieghere, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D71398