[index] Enhance indexing for module references

* Create a USR for the occurrences of the 'module' symbol kind
* Record module references for each identifier in an import declaration

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@342484 91177308-0d34-0410-b5e6-96231b3b80d8
12 files changed