Example log:
```
15:47:45 indexer1 pipeline.cc:379 I parse /tmp/d/a.c error:1 use of undeclared identifier 'arg'
clang /tmp/d/a.c --gcc-toolchain=/usr -working-directory=/tmp/d/
```
Fix a minor issue that an empty included file is not recorded.
Note, we need to skip invalid uid2lid_and_path because
otherwise lid2path may contain invalid entries with lid: 0,
which will cause the file entry with file_id 0 to have an empty path.
By default, the background indexer doesn't handle names of no linkage.
They are indexed when their files are opened. This saves memory and
makes cache files smaller.
* In the "initialized" callback, send client/registerCapability with DidChangeWatchedFilesRegistrationOptions
* In workspace/didChangeWatchedFiles callback, call pipeline::Index
* In pipeline::Index, add a `deleted` status
* WorkingFiles::files : vector -> unordered_map
* Add timestamp to WorkingFile
* Rename "comp-preload" thread to "preamble"
* Rename CompletionManager to SemaManager as it is used by "diag" "comp" "preamble"
* Rename clang_complete.* to sema_manager.*
* Merge SemaManager::{preloads,sessions}
* Add initialization option session.maxNum
* In DiagnosticMain, if an included file was modified, cancel the DiagTask and create a PreambleTask instead. The task sets `from_diag` so as to trigger immediate DiagTask after the preamble is built.
