Do not warn about format strings that are indexed string literals.

Summary:
The warning for a format string not being a string literal and therefore
being potentially insecure is overly strict for indices into string
literals. This fix checks if the index into the string literal is
precomputable. If that's the case it will check if the suffix of that
string literal is a valid format string string literal. It will still
issue the aforementioned warning for out of range indices into the
string literal.

Patch by Meike Baumgärtner (meikeb)

Reviewers: rsmith

Subscribers: srhines, cfe-commits

Differential Revision: https://reviews.llvm.org/D24584

llvm-svn: 281686