[clangd] Remove accessors for top-level decls from preamble

Summary:
They cause lots of deserialization and are not actually used.
The ParsedAST accessor that previously returned those was renamed from
getTopLevelDecls to getLocalTopLevelDecls in order to avoid
confusion.

This change should considerably improve the speed of findDefinition
and other features that try to find AST nodes, corresponding to the
locations in the source code.

Reviewers: ioeric, sammccall

Reviewed By: sammccall

Subscribers: klimek, mehdi_amini, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D47331

llvm-svn: 333371