[clangd] Support clang-tidy configuration in clangd.

Summary:
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

llvm-svn: 351792