Modified format-string checking to not emit a warning when all of the

Modified format-string checking to not emit a warning when all of the
following hold:

(1) A vprintf-like function is called that takes the argument list via a
    via_list argument.

(2) The format string is a non-literal that is the parameter value of
    the enclosing function, e.g:

    void logmessage(const char *fmt,...) {
      va_list ap;
      va_start(ap,fmt);
      fprintf(fmt,ap);  // Do not emit a warning.
    }

In the future this special case will be enhanced to consult the "format"
attribute attached to a function declaration instead of just allowing a blank
check for all function parameters to be used as format strings to vprintf-like
functions. This will happen when more support for attributes becomes
available.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@45114 91177308-0d34-0410-b5e6-96231b3b80d8