[ASTImporter] Fix import of lambda in function param

Summary:
The current import implementation fails to import the definition of a
lambda class if the lambda class is defined in a function param.
E.g., the lambda class below will be imported without any methods:
```
  template <typename F>
  void f(F L = [](){}) {}
```

Reviewers: a_sidorin, a.sidorin, shafik

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64073

llvm-svn: 365315