Reduce false positives in printf/scanf format checker

Reduce false positives in printf/scanf format checker

Summary:
The printf/scanf format checker is a little over-zealous in handling the conditional operator.  This patch reduces work by not checking code-paths that are never used and reduces false positives regarding uncovered arguments, for example in the code fragment:

printf(minimal ? "%i\n" : "%i: %s\n", code, msg);

Reviewers: rtrieu

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D15636

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@262025 91177308-0d34-0410-b5e6-96231b3b80d8