[ASTImporter] Do not look up lambda classes

Summary:
Consider this code:
```
      void f() {
        auto L0 = [](){};
        auto L1 = [](){};
      }

```
First we import `L0` then `L1`. Currently we end up having only one
CXXRecordDecl for the two different lambdas. And that is a problem if
the body of their op() is different. This happens because when we import
`L1` then lookup finds the existing `L0` and since they are structurally
equivalent we just map the imported L0 to be the counterpart of L1.

We have the same problem in this case:
```
      template <typename F0, typename F1>
      void f(F0 L0 = [](){}, F1 L1 = [](){}) {}

```

In StructuralEquivalenceContext we could distinquish lambdas only by
their source location in these cases. But we the lambdas are actually
structrually equivalent they differn only by the source location.

Thus, the  solution is to disable lookup completely if the decl in
the "from" context is a lambda.
However, that could have other problems: what if the lambda is defined
in a header file and included in several TUs? I think we'd have as many
duplicates as many includes we have. I think we could live with that,
because the lambda classes are TU local anyway, we cannot just access
them from another TU.

Reviewers: a_sidorin, a.sidorin, shafik

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66348

llvm-svn: 370461