llvm-gsymutil now handles empty linkage names correctly. (#68931)

Previous to this fix, if we had a DW_TAG_subprogram that had a
DW_AT_linkage_name that was empty, it would attempt to use this name
which would cause an error to be emitted when saving the gsym file to
disk:

error: DWARF conversion failed: : attempted to encode invalid
FunctionInfo object

This patch fixes this issue and adds a unit test case.