[clangd] Look for compilation database in `build` subdirectory of parents.

Summary:
This matches the conventional location of cmake build directories.
It also allows creating a `build` symlink, which seems more flexible than the
compile_commands.json symlinks.

Reviewers: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78629