[clangd] Show non-instantiated decls in signatureHelp

Summary:
To avoid producing very verbose output in substitutions involving
typedefs, e.g.
  T -> std::vector<std::string>::iterator
gets turned into an unreadable mess when printed out for libstdc++,
result contains internal types (std::__Vector_iterator<...>) and
expanded well-defined typedefs (std::basic_string<char>).

Until we improve the presentation code in clang, going with
non-instantiated decls looks like a better UX trade-off.

Reviewers: hokein, ioeric, kadircet

Reviewed By: hokein

Subscribers: MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D50645

llvm-svn: 339665