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

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

Reviewers: rsmith

Subscribers: srhines, cfe-commits

Differential Revision: https://reviews.llvm.org/D23820

llvm-svn: 281527