[clangd] Store references instead of unique_ptrs in ClangdServer.

Summary:
ClangdServer owned objects passed to it in constructor for no good reason.
Lots of stuff was moved from the heap to the stack thanks to this change.

Reviewers: krasimir

Reviewed By: krasimir

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D34148

llvm-svn: 305298