Emit static constexpr member as available_externally definition

By exposing the constant initializer, the optimizer can fold many
of these constructs.

This is a recommit of r311857 that was reverted in r311898 because
an assert was hit when building Chromium.
We have to take into account that the GlobalVariable may be first
created with a different type than the initializer. This can
happen for example when the variable is a struct with tail padding
while the initializer does not have padding. In such case, the
variable needs to be destroyed an replaced with a new one with the
type of the initializer.

Differential Revision: https://reviews.llvm.org/D34992

llvm-svn: 312512