[analyzer] Rename clang::CallGraph into clang::idx::CallGraph + rename

the corresponding files to avoid confusion.

This is a preparation to adding an AST-based call graph to Analysis. The
existing call graph works with indexer entries. We might be able to
refactor it to use the AST based graph in the future.

(Minimal testing here as the only example that uses the API has been
completely broken, does not compile.)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@151950 91177308-0d34-0410-b5e6-96231b3b80d8