[analyzer] Add basic format string vulnerability checking.

We already have a more conservative check in the compiler (if the
format string is not a literal, we warn). Still adding it here for
completeness and since this check is stronger - only triggered if the
format string is tainted.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@147714 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed