[clangd] Drop namespace references in the index.

Summary:
Namespace references is less useful compared with other symbols, and
they contribute large part of the index. This patch drops them.
The number of refs is reduced from 5.4 million to 4.7 million.

|           |  Before | After |
|file size  |  78 MB  |  71MB |
|memory     |  330MB  |  300MB|

Reviewers: sammccall

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

Differential Revision: https://reviews.llvm.org/D54202

llvm-svn: 346319