[CodeComplete] Add an option to omit results from the preamble.

Summary:
Enumerating the contents of a namespace or global scope will omit any
decls that aren't already loaded, instead of deserializing them from the
PCH.

This allows a fast hybrid code completion where symbols from headers are
provided by an external index. (Sema already exposes the information
needed to do a reasonabl job of filtering them).
Clangd plans to implement this hybrid.

This option is just a hint - callers still need to postfilter results if
they want to *avoid* completing decls outside the main file.

Reviewers: bkramer, ilya-biryukov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D41989

llvm-svn: 322371