Enable std::underlying_type for gcc >= 4.7

[Albert J. Wong (王重傑)]

Hi Marshall,

gcc 4.7 added enough frontend support to implement most (not all) of
type_traits. Here's a patch that enables std::underlying_type.

FYI, this is a first in a series of ~3 that will allow most libc++ tests
for type_traits and atomics to pass on gcc >= 4.7.  Will send those out as
I manage to get them fully cleaned up.

Thanks,
Albert
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