[clangd] Allow using customized include path in URI.

Summary:
Calculating the include path from absolute file path does not always
work for all build system, e.g. bazel uses symlink as the build working
directory. The absolute file path from editor and clang is diverged from
each other. We need to address it properly in build sysmtem integration.

This patch worksarounds the issue by providing a hook in URI which allows
clients to provide their customized include path.

Reviewers: sammccall

Subscribers: klimek, ilya-biryukov, jkorous-apple, ioeric, MaskRay, cfe-commits

Differential Revision: https://reviews.llvm.org/D45426

llvm-svn: 329578