[clangd] Uprank delcarations when "using q::name" is present in the main file

Having `using qualified::name;` for some symbol is an important signal
for clangd code completion as the user is more likely to use such
symbol.  This patch helps to uprank the relevant symbols by saving
UsingShadowDecl in the new field of CodeCompletionResult and checking
whether the corresponding UsingShadowDecl is located in the main file
later in ClangD code completion routine. While the relative importance
of such signal is a subject to change in the future, this patch simply
bumps DeclProximity score to the value of 1.0 which should be enough for
now.

The patch was tested using

`$ ninja check-clang check-clang-tools`

No unexpected failures were noticed after running the relevant testsets.

Reviewers: sammccall, ioeric

Subscribers: MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D49012

llvm-svn: 336810