This patch implements the functionality specified by DR948.
The changes are two fold. First, the parser was modified
to allow 'constexpr's to appear in condition declarations
(which was a hard error before). Second, Sema was modified
to cleanup maybe odr-used declarations. As 'constexpr's
were not allowed in condition declarations before the cleanup
wasn't necessary (such declarations were always odr-used).
This fixes PR22491.