[Analysis] Validate the return type of s(n)printf like libcalls

If the sprintf function is static (as on mingw-w64, where many stdio
functions are static inline wrappers), earlier optimization passes
could optimize out the return value altogether, and make it void,
which could break optimizations of this libcall that touch the
return value.

This fixes the issue discussed in PR37408 for the sprintf function.

Differential Revision: https://reviews.llvm.org/D46752

llvm-svn: 332106