[clangd] Add hot-reload of compile_commands.json and compile_flags.txt
When querying the CDB, we stat the underlying file to check it hasn't changed.
We don't do this every time, but only if we didn't check within 5 seconds.
This behavior only exists for compile_commands.json and compile_flags.txt.
The CDB plugin system doesn't expose enough information to handle others.
Slight behavior change: we now only look for build/compile_commands.json
rather than trying every CDB strategy under build subdirectories.
Differential Revision: https://reviews.llvm.org/D92663