FileDB wraps the access to the contents of project files.
When asked to fetch a file, FileDB will either return its in-memory
model of the file if it has been "opened" by the language-server
protocol, or otherwise fetch it from the filesystem.
Previously, the cpp language server did not pledge "rpath" and got
access to the contents of files whenever they were opened by the
language client.
However, features like inspection of header files require the language
server to get the content of files that were not opened by the client.
The language server now pledges rpath but makes sure to only unveil
the project's directory and /usr/include.
Previously, if a new LanguageClient was created & destroyed, the
ServerConnection to the language server would be left without an
attached LanguageClient.
As a result, auto-completion results would not be updated in the UI.
Starting with this commit, the LanguageClient holds a WeakPtr to the
previous LanguageClient that was attached to the ServerConnection,
and re-attaches it after detaching itself.
