[CodeComplete] Don't replace the rest of line in #include completion.

Summary:
The previous behavior was aggressive,
  #include "abc/f^/abc.h"
                foo/  -> candidate
"f/abc.h" is replaced with "foo/", this patch will preserve the "abc.h".

Reviewers: sammccall

Subscribers: jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76770