[ASTImporter] Fix unhandled cases in ASTImporterLookupTable

Summary:
In most cases the FriendDecl contains the declaration of the befriended
class as a child node, so it is discovered during the recursive
visitation. However, there are cases when the befriended class is not a
child, thus it must be fetched explicitly from the FriendDecl, and only
then can we add it to the lookup table.
(Note, this does affect only CTU and does not affect LLDB, because we
cannot and do not use the ASTImporterLookupTable in LLDB.)

Reviewers: a_sidorin, a.sidorin, shafik

Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62064

llvm-svn: 363062