[clangd] Respect Origin option in createStaticIndexingAction

Summary:
Currently createStaticIndexingAction always set Origin to Static, which
makes it hard to change it later on by different indexers(One needs to go over
each symbol making a new copy).

This patch changes that behavior to rather respect it if set by user.

Reviewers: ioeric

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59205

llvm-svn: 355820