[clangd] Add callbacks on parsed AST in addition to parsed preambles

Summary:
Will be used for updating the dynamic index on updates to the open files.
Currently we collect only information coming from the preamble
AST. This has a bunch of limitations:
  - Dynamic index misses important information from the body of the
    file, e.g. locations of definitions.
  - XRefs cannot be collected at all, since we can only obtain full
    information for the current file (preamble is parsed with skipped
    function bodies, therefore not reliable).

This patch only adds the new callback, actually updates to the index
will be done in a follow-up patch.

Reviewers: hokein

Reviewed By: hokein

Subscribers: kadircet, javed.absar, ioeric, MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D50847

llvm-svn: 340401