[clangd] Support clang-tidy configuration in clangd.
This patch adds some basic supports for clang-tidy configurations in clangd:
- clangd will respect .clang-tidy configurations for each file
- we don't aim to support all clang-tidy options in clangd, only a small subset of condfigurations (options related to which checks will be enabled) are supported.
- add a clang-tidy-checks CLI option that can override options from .clang-tidy file
Reviewers: ilya-biryukov, sammccall
Reviewed By: sammccall
Subscribers: javed.absar, ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits
Differential Revision: https://reviews.llvm.org/D55256