If the size of a global variable is not known at compile time, we emit
a fixed size buffer together with initialization code when the global
variable is initialized.
Make sure the SIL optimizer does not convert this into a static
initialization, even if the size of the type is known inside the
module where the global is declared, because we don't have a way to
statically initialize something that looks like a fixed-size buffer
to client code.
Fixes <https://bugs.swift.org/browse/SR-11709>.
