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 by way of a call to
'ActOnFinishFullExpr'. As 'constexpr's were not allowed in
condition declarations before the cleanup wasn't necessary
(such declarations were always odr-used).
This fixes PR22491.
Differential Revision: http://reviews.llvm.org/D8978
llvm-svn: 240707