Debug Info: generate a unique identifier for C++ struct, class, union, and enum.

We use CXX mangler to generate unique identifier for external C++ struct,
union, class and enum. Types with unique identifier are added to RetainedTypes
to make sure they are treated as used even when all uses are replaced with
the identifiers.

A single type can be added to RetainedTypes multiple times. For example, both
createForwardDecl and createLimitedType can add the same type to RetainedTypes.
A set is used to avoid duplication when updating AllRetainTypes in DIBuilder.

Testing cases are updated to reflect the unique identifier generated for types.
The order of MDNodes is changed because of retained types and testing cases
are updated accordingly.

Testing case debug-info-uuid.cpp now emits error with Itanium mangler, since
uuid is not yet handled in Itanium mangler.

We choose to update RetainedTypes in clang, then at finalize(), we update
AllRetainTypes in DIBuilder. The other choice is to update AllRetainTypes
in DIBuilder when creating a DICompositeType with unique identifier. This
option requires using ValueHandle for AllRetainTypes in DIBuilder since
the created DICompositeType can be modified later on by setContainingType etc.