[ASTImporter] Friend class decl should not be visible in its context
In the past we had to use DeclContext::makeDeclVisibleInContext to make
friend declarations available for subsequent lookup calls and this way
we could chain (redecl) the structurally equivalent decls.
By doing this we created an AST that improperly made declarations
visible in some contexts, so the AST was malformed.
Since we use the importer specific lookup this is no longer necessary,
because with that we can find every previous nodes.
Reviewers: balazske, a_sidorin, a.sidorin, shafik
Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, teemperor, cfe-commits
Differential Revision: https://reviews.llvm.org/D71020