[clangd] Boost code completion results that were named in the last few lines.

Summary:
The hope is this will catch a few patterns with repetition:

  SomeClass* S = ^SomeClass::Create()

  int getFrobnicator() { return ^frobnicator_; }

  // discard the factory, it's no longer valid.
  ^MyFactory.reset();

Without triggering antipatterns too often:

  return Point(x.first, x.^second);

I'm going to gather some data on whether this turns out to be a win overall.

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, jfb, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61537

llvm-svn: 360030