[strictfp] Replace dangling strictfp attrs with nobuiltin

In preparation for a patch that will enforce new rules for the usage of
the strictfp attribute, this patch introduces auto-upgrade behavior that
will replace the strictfp attribute on callsites with nobuiltin if the
enclosing function declaration doesn't also have the strictfp attribute.

This auto-upgrade isn't being performed on .ll files because that would
prevent us from writing a test for the forthcoming verifier behavior.

Differential Revision: https://reviews.llvm.org/D70096