[clangd] Don't rename the namespace.

Summary:
Also fix a small bug -- the extra argument "-xc++" doesn't overwrite the
language if the argument is present after the file name in the compiler
command.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63759

llvm-svn: 364392