Actually, I think I've figured it out. It looks like bugs 770025, which
was an extension of bug 736607, were intended to force gcc-4.7 to be
upgraded to the Jessie version. However, these changes were then trumped
by bug 765379 which just straight-up removed gcc-4.7 from Debian Jessie.
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=770025
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=736607
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=765379
I've opened a new bug to address this issue.
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=798029
