[clangd] Move caching of compile args out of ClangdServer.

Summary:
Caching is now handled by ClangdLSPServer and hidden behind the
GlobalCompilationDatabase interface. This simplifies ClangdServer.
This change also removes the SkipCache flag from addDocument,
which is now obsolete.

No behavioral changes are intended, the clangd binary still caches the
compile commands on the first read.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: mgorny, ioeric, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D48068

llvm-svn: 334585