Add placeholder function in Sema for new format string checking logic.

Add placeholder function in Sema for new format string checking logic.
This function will use the format string parsing logic in libAnalysis,
and once it is shown to be better than the current implementation it
will replace AlternateCheckPrintfString() entirely.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@94721 91177308-0d34-0410-b5e6-96231b3b80d8