[clangd] Allow observation of changes to global CDBs.

Summary:
Currently, changes *within* CDBs are not tracked (CDB has no facility to do so).
However, discovery of new CDBs are tracked (all files are marked as modified).
Also, files whose compilation commands are explicitly set are marked modified.

The intent is to use this for auto-index. Newly discovered files will be indexed
with low priority.

Reviewers: ilya-biryukov

Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D54475

llvm-svn: 347297