* Better LanguageId detection with clangDriver (e.g. .cu -> types::TY_CUDA)
* fallback when there is no .ccls or compile_commands.json
Also Hide clangTooling options from --help
Move using below #include to make preamble happy
textDocument/references: if no references, first line or last line => list where this file is included
malloc_trim() only if files have been indexed in last cycle
Intern: use CachedHashStringRef