[ASTImporter] Fix name conflict handling with different strategies


[ASTImporter] Fix name conflict handling with different strategies

There are numorous flaws about the name conflict handling, this patch
attempts fixes them. Changes in details:

  • HandleNameConflict return with a false DeclarationName

Hitherto we effectively never returned with a NameConflict error, even
if the preceding StructuralMatch indicated a conflict.
Because we just simply returned with the parameter Name in
HandleNameConflict and that name is almost always true when converted to

  • Add tests which indicate wrong NameConflict handling
  • Add to ConflictingDecls only if decl kind is different

Note, we might not indicate an ODR error when there is an existing record decl
and a enum is imported with same name. But there are other cases. E.g. think
about the case when we import a FunctionTemplateDecl with name f and we found a
simple FunctionDecl with name f. They overload. Or in case of a
ClassTemplateDecl and CXXRecordDecl, the CXXRecordDecl could be the 'templated'
class, so it would be false to report error. So I think we should report a
name conflict error only when we are 100% sure of that. That is why I think it
should be a general pattern to report the error only if the kind is the same.

  • Fix failing ctu test with EnumConstandDecl

In ctu-main.c we have the enum class 'A' which brings in the enum
constant 'x' with value 0 into the global namespace.
In ctu-other.c we had the enum class 'B' which brought in the same name
('x') as an enum constant but with a different enum value (42). This is clearly
an ODR violation in the global namespace. The solution was to rename the
second enum constant.

  • Introduce ODR handling strategies

Reviewers: a_sidorin, shafik

Differential Revision: https://reviews.llvm.org/D59692