[DebugInfo] Generate debug information for labels. (After fix PR39094)

Generate DILabel metadata and call llvm.dbg.label after label
statement to associate the metadata with the label.

After fixing PR37395.
After fixing problems in LiveDebugVariables.
After fixing NULL symbol problems in AddressPool when enabling
split-dwarf-file.
After fixing PR39094.

Differential Revision: https://reviews.llvm.org/D45045

llvm-svn: 345009