[clangd] Customizable URI schemes for dynamic index.

Summary:
This allows dynamic index to have consistent URI schemes with the
static index which can have customized URI schemes, which would make file
proximity scoring based on URIs easier.

Reviewers: sammccall

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D47931

llvm-svn: 334809