[clang][ASTImporter]Skip check depth of friend template parameter

Depth of the parameter of friend template class declaration in a
template class is 1, while in the specialization the depth is 0.
This will cause failure on 'IsStructurallyEquivalent' as a name
conflict in 'VisitClassTemplateDecl'(see testcase of
'SkipComparingFriendTemplateDepth'). The patch fix it by ignore
the depth only in this special case.

Reviewed By: balazske

Differential Revision: https://reviews.llvm.org/D156693