[clangd] Make system header mappings available for PreambleParsedCallback

Summary:
SystemHeaderMappings were added only after takeIncludes call, which
resulted in getting mapping on main file ast updates but not on preamble ast
updates.
Fixes https://github.com/clangd/clangd/issues/8

Reviewers: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58029