[libclang] Simplify the indexing API.

[libclang] Simplify the indexing API.

Cut down the number of callbacks to more generic ones. Clients can check
an enum to find out what kind of declaration it is and they can call functions
to get more specific information than the generic provided info.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@144343 91177308-0d34-0410-b5e6-96231b3b80d8