Implement a general type scavenger that can dig types from debug info + a...

Implement a general type scavenger that can dig types from debug info + a filtering mechanism to accept/reject results thusly obtained

Implement the C++ type lookup support in terms of this general scavenger

The idea is that we may want other languages to do debug info based search (exclusively, or as an add-on to runtime/module based searching) and it makes sense to avoid duplicating this functionality

llvm-svn: 285727