[clangd] Add clang-tidy options to config
First step of implementing clang-tidy configuration into clangd config.
This is just adding support for reading and verifying the clang tidy options from the config fragments.
No support is added for actually using the options within clang-tidy yet.
That will be added in a follow up as its a little more involved.
Reviewed By: sammccall
Differential Revision: https://reviews.llvm.org/D90531