[analyzer] CStringChecker: Remember to highlight the argument expression range.

[analyzer] CStringChecker: Remember to highlight the argument expression range.

When emitting a bug report, it is important to highlight which argument of the
call-expression is causing the problem.

Before:
warning: Null pointer argument in call to string comparison function
  strcmp(a, b);
  ^~~~~~~~~~~~

After:
warning: Null pointer argument in call to string comparison function
  strcmp(a, b);
  ^      ~

Affects other output modes as well, not just text.

Differential Revision: https://reviews.llvm.org/D50028

llvm-svn: 338333