[clangd] Support multiple #include headers in one symbol.

Summary:
Currently, a symbol can have only one #include header attached, which
might not work well if the symbol can be imported via different #includes depending
on where it's used. This patch stores multiple #include headers (with # references)
for each symbol, so that CodeCompletion can decide which include to insert.

In this patch, code completion simply picks the most popular include as the default inserted header. We also return all possible includes and their edits in the `CodeCompletion` results.

Reviewers: sammccall

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D51291

llvm-svn: 341304