[clangd] Some nitpicking around the new split (preamble/main) dynamic index

Summary:
- DynamicIndex doesn't implement ParsingCallbacks, to make its role clearer.
  ParsingCallbacks is a separate object owned by the receiving TUScheduler.
  (I tried to get rid of the "index-like-object that doesn't implement index"
  but it was too messy).
- Clarified(?) docs around DynamicIndex - fewer details up front, more details
  inside.
- Exposed dynamic index from ClangdServer for memory monitoring and more
  direct testing of its contents (actual tests not added here, wanted to get
  this out for review)
- Removed a redundant and sligthly confusing filename param in a callback

Reviewers: ilya-biryukov

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

Differential Revision: https://reviews.llvm.org/D51221

llvm-svn: 341325