Rename libIDE to libIndex.

Per feedback from Chandler, it's better to have libraries with more specific functionality.
LibIndex will contain the indexing functionality of libclang, which includes USR generation.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188601 91177308-0d34-0410-b5e6-96231b3b80d8