-Wformat: Don't check format strings in uninstantiated templates.

Also applies to -Wnonnull, -Wtype-safety, and -Wnon-pod-varargs.
All of these can be better checked at instantiation time.

This change does not actually affect regular CallExpr function calls,
since the checks there only happen after overload resolution.
However, it will affect Objective-C method calls.

<rdar://problem/12373934>

llvm-svn: 164984