[ASTImporter] Support importing CXXUnresolvedConstructExpr and UnresolvedLookupExpr

* Note: This solution is based on
  https://github.com/haoNoQ/clang/blob/summary-ipa-draft/lib/AST/ASTImporter.cpp#L7605.

Patch by Peter Szecsi!

Differential Revision: https://reviews.llvm.org/D38694

llvm-svn: 322091