When performing IRGen on a global, emit it as a constant if:

 1) It has a const-qualified type, and
 2) It has no mutable members, and
 3) It has no dynamic initialization, and
 4) It has trivial destruction.
Remove the unnecessary requirement that the type be POD. This allows us to
mark all constexpr objects with no mutable members as 'constant'.

llvm-svn: 150792