extra: %U support in time::strftime

Fixes #2350
avoid alloca in with_overflow intrinsics

This is pointless, since it's just used to load an `undef` value and
never touched again.
extra: %G, %g and %V support in time::strftime

Fixes #2350
Check enums in missing_doc lint

Closes #9671
auto merge of #9670 : luisbg/rust/strftime, r=alexcrichton
auto merge of #9680 : thestinger/rust/overflow, r=huonw

This is pointless, since it's just used to load an `undef` value and
never touched again.
auto merge of #9675 : sfackler/rust/lint, r=alexcrichton

Closes #9671