This feels like a better name since the "autocomplete engine" can, in
addition to providing autocomplete suggestions, also find declarations
of symbols and report back the symbols that are defined in a document.
Also, Cpp/ParserAutoComplete has been renamed to CppComprehensionEngine
and Shell/AutoComplete has been renamed to ShellComprehensionEngine.
This makes them available for use by other language servers.
Also as a bonus, update the Shell language server to discover some
symbols and add go-to-definition functionality :^)
