[windows-itanium] Propagate DLL storage class to Initialisation Guard Variables

Initialisation Guard Variables should take their DLL storage class
from the guarded variable. Otherwise, there will be a link error if
the compiler inlines a reference to the guard variable into another
module but that guard variable is not exported from the defining
module.

This is required for platforms such as PlayStation and
windows-itanium, that are aiming for source compatibility with MSVC
w.r.t. dllimport/export annotations, given Clang's existing design
which allows for inlining of a dllimport function as long as all the
variables/functions referenced are also marked dllimport.

A similar change exists for the MSVC ABI:
https://reviews.llvm.org/D4136.

I have added a run test for windows-itanium for this issue to the
build recipe: https://reviews.llvm.org/D88124.

Differential Revision: https://reviews.llvm.org/D138463