[include-fixer] Be smarter about inserting symbols for a prefix.

If prefix search finds something where nothing can be nested under (e.g.
a variable or macro) don't add it to the result.

This is for cases like:
header.h:
  extern int a;

file.cc:
namespace a {
  SOME_MACRO
}

We will look up a::SOME_MACRO, which doesn't have any results. Then we
look up 'a' and find something before we ever look up just 'SOME_MACRO'.
With some basic filtering we can avoid this case.

Differential Revision: http://reviews.llvm.org/D20960

llvm-svn: 271671