[patch] Update LWG issues lists and implementation status

[Jonathan Wakely]

This backports a doc change to the gcc-5-branch. The patch doesn't
include the updated issues lists, since they're just copied from
upstream and are already on trunk and gcc-6-branch.

The list of implemented DRs is slightly different to the one on trunk,
because not all of them are fixed on gcc-5-branch. I think I got the
list right for the branch status.

Committed to gcc-5-branch.


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