[clangd] Put direct headers into srcs section.

Summary:
Currently, there's no way of knowing about header files
using compilation database, since it doesn't contain header files as entries.

Using this information, restoring from cache using compile commands becomes
possible instead of doing directory traversal. Also, we can issue indexing
actions for out-of-date headers even if source files depending on them haven't
changed.

Reviewers: sammccall

Subscribers: ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D54817

llvm-svn: 347669