[ASTImporter] Add importer specific lookup

Summary:
There are certain cases when normal C/C++ lookup (localUncachedLookup)
does not find AST nodes. E.g.:

Example 1:

  template <class T>
  struct X {
    friend void foo(); // this is never found in the DC of the TU.
  };

Example 2:

  // The fwd decl to Foo is not found in the lookupPtr of the DC of the
  // translation unit decl.
  struct A { struct Foo *p; };

In these cases we create a new node instead of returning with the old one.
To fix it we create a new lookup table which holds every node and we are
not interested in any C++ specific visibility considerations.
Simply, we must know if there is an existing Decl in a given DC.

Reviewers: a_sidorin, a.sidorin

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

Differential Revision: https://reviews.llvm.org/D53708

llvm-svn: 349351