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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240707 91177308-0d34-0410-b5e6-96231b3b80d8