[flang] Create HostAssoc symbols for uplevel references

To make it easier for lowering to identify which symbols from the host
are captured by internal subprograms, create HostAssocDetails for them.

In particular, if a symbol is referenced and it is contained in a
subprogram or main program that is not the same as the containing
program unit of the reference, a HostAssocDetails symbol is created
in the current scope.

Differential Revision: https://reviews.llvm.org/D84889