[clangd] #include statements support for Open definition

Summary: ctrl-clicking on #include statements now opens the file being pointed by that statement.

Reviewers: malaperle, krasimir, bkramer, ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: jkorous-apple, ioeric, mgrang, klimek, ilya-biryukov, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D38639

llvm-svn: 325662