Revert r311857 "Emit static constexpr member as available_externally definition"

It caused PR759744.

> Emit static constexpr member as available_externally definition
>
> By exposing the constant initializer, the optimizer can fold many
> of these constructs.
>
> Differential Revision: https://reviews.llvm.org/D34992

llvm-svn: 311898