[Type Uniquing Patch] Use identifier instead of MDNode for getClassType of ptr_to_member.

[Manman Ren]

Also add a typedef for DITypeIdentifierMap and a helper
generateDITypeIdentifierMap in DebugInfo.h. In DwarfDebug.cpp, we keep
a DITypeIdentifierMap and call generateDITypeIdentifierMap to actually
populate the map. DwarfDebug also has a helper function lookupType that
finds the type MDNode for a given Value (either an identifier in MDString
or the actual MDNode).

Verifier is updated accordingly.

3 patches are attached:
The first one is a simple refactoring to add getField to DIDescriptor so we
can use it to return Value* for getClassType.

The second one is the actual change to DIBuilder and DwarfDebug etc with an
added testing case.

The last one is to update testing cases in clang.

Thanks,
Manman
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