[Type Uniquing Patch] Modify functions in DebugInfo.h to take an extra map argument.

[Manman Ren]

Modify access functions that can return a DIType to take an extra map argument.
Add helper function
+  void collectDITypeHashMap(const NamedMDNode *CU_Nodes, DITypeHashMap &Map);
to update DITypeHashMap by going through RetainedTypes of each CU.
Testing cases are added to verify llvm-link can correctly unique types if the input .ll files use string instead of DIType.

To make sure we can dump MDNode comments even without a DITypeHashMap, isDebugInfo is used instead of
Verify in WriteMDNodeComment. Neither is accurate for checking whether MDNode is indeed debug info.

Thanks,
Manman

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