[PATCH] D157114: [clang][ASTImporter] Improve StructuralEquivalence algorithm on repeated friends

[Ding Fei via Phabricator via cfe-commits]

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Repeated friends are deduplicated when imported, but StructuralEquivalence checks friends by exact matching.
If `ToContext` is empty (not containing the class to be imported), the imported friends are deduplicated, any
further importing of the class would be rejected at the structure equivalence checking, i.e:

  struct foo { friend class X; friend class X; }; // FromContext

only one friend is imported, similar to the following:

  struct foo { friend class X; }; // ToContext

but when imported again, `struct foo` in FromContext is reported as not equivalent to `struct foo` in `ToContext`,
thus rejected.

The structural equivalence checking algorithm is improved by applying similar deduplication as Importer does.
Thus from StructuralEquivalence's point of view, the above two `RecordDecl`s are equivalent.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D157114

Files:
  clang/lib/AST/ASTImporter.cpp
  clang/lib/AST/ASTStructuralEquivalence.cpp
  clang/unittests/AST/ASTImporterTest.cpp
  clang/unittests/AST/StructuralEquivalenceTest.cpp

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