[clangd] Add std subnamespace symbols to the symbol map.

Reviewers: ioeric, serge-sans-paille

Reviewed By: ioeric

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59364

llvm-svn: 356894