[clangd] Support passing a relative path to -compile-commands-dir

Summary: This is useful when using clangd with CMake based projects in Visual Studio Code since when using CMake the `compile_commands.json` file is usually located in a `build` subdirectory which isn't a parent directory of the source files. Allowing passing relative paths to -compile-commands-dir allows specifying `clangd.arguments = ["-compile-commands-dir=build"]` in VSCode's settings file and having it work for each CMake based project that uses the `build` subdirectory as the build directory (instead of having to specify the absolute path to the compile commands directory for each separate project in VSCode's settings).

Patch by Daan De Meyer!

Reviewers: sammccall

Reviewed By: sammccall

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

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D53481

llvm-svn: 345022