[clang][ASTImporter] Fix import of typedef with unnamed structures

Fix crash in ASTImporter related to import of unnamed structures and typedefs
to these maybe with pointer.
There was a series of problems exposed by https://reviews.llvm.org/D133468
(commit 69a64174) in the ASTImporter breaking
cross-translation unit analysis. This change fixes one of the problems exposed
by that change for importing unnamed structures. The problem was
discovered when running clang static analysis on open source projects
using cross-translation unit analysis.

Simple test command. Produces crash without change, passes all tests
with change.

```
ninja ASTTests && ./tools/clang/unittests/AST/ASTTests
  --gtest_filter="*/*ImportAnonymousStruct/0"
```

Formatted crash stack:

```
ASTTests: <root>/clang/lib/AST/ASTContext.cpp:4787:
  clang::QualType clang::ASTContext::getTypedefType(const clang::TypedefNameDecl*,
  clang::QualType) const: Assertion `hasSameType(Decl->getUnderlyingType(), Underlying)' failed.
...
 #9 <addr> clang::ASTContext::getTypedefType(clang::TypedefNameDecl const*, clang::QualType) const
             <root>/clang/lib/AST/ASTContext.cpp:4789:26
             <root>/clang/lib/AST/ASTImporter.cpp:1374:71
             <root>/tools/clang/include/clang/AST/TypeNodes.inc:75:1
             <root>/clang/lib/AST/ASTImporter.cpp:8663:8
```

Reviewed By: donat.nagy

Differential Revision: https://reviews.llvm.org/D145868