For better compatibility with C++11 and C++14, emit a nondiscardable definition

of a static constexpr data member if it's defined 'constexpr' out of line, not
only if it's defined 'constexpr' in the class.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@316310 91177308-0d34-0410-b5e6-96231b3b80d8