[flang][MIF] Fix undef reference to a coarray_handle in mif.dealloc_coarray #193157 (#213890)
This PR fixes the behavior reported in issue #193157. The coarray_handle
was only defined if a call to mif.alloc_coarray was present.
If a call to mif.dealloc_coarray was encountered without a prior call to
mif.alloc_coarray, then the coarray_handle was missing, and therefore
llvm.address_of pointed to a non-existent address, which is not allowed.
We now define a coarray_handle that has not been allocated by PRIF for
each coarray variables.