Introduce the 'Index' library.

Its purpose is to provide the basic infrastructure for cross-translation-unit analysis like indexing, refactoring, etc.

Currently it is very "primitive" and with no type-names support. It can provide functionality like
"show me all references of this function from these translation units".

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@74802 91177308-0d34-0410-b5e6-96231b3b80d8
13 files changed