[Libomptarget] Use entry name for global info

Currently, globals on the device will have an infinite reference count
and an unknown name when using `LIBOMPTARGET_INFO` to print the mapping
table. We already store the name of the global in the offloading entry
so we should be able to use it, although there will be no source
location. To do this we need to create a valid `ident_t` string from a
name only.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D124381