[clangd] Check for include overlapping looks for only the line now.

Summary:
Currently we match an include only if we are inside filename, with this patch we
will match whenever we are on the starting line of the include.

Reviewers: ilya-biryukov, hokein, ioeric

Reviewed By: ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D51163

llvm-svn: 340539