o Index interfaces to support using different index sources (e.g. AST index, global index) for code completion, cross-reference finding etc. This patch focuses on code completion.
The following changes in the original patch has been split out.
o Implement an AST-based index.
o Add an option to replace sema code completion for qualified-id with index-based completion.
o Implement an initial naive code completion index which matches symbols that have the query string as substring.