[ASTImporter] Fix isa cast assert

[ASTImporter] Fix isa cast assert

Do early return if we can't import the found decl for a member expr.
This follows the pre-existing scheme, e.g with E->getMemberDecl().
E->getFoundDecl().getDecl() can be null when a member expression does
not involve lookup. It may involve a lookup in case of a using directive
which refers to a member function in a base class template.

We faced this assert during the CTU analysis of google::protobuf v3.5.2.
We tried hard to synthesize a minimal test example both by hand and by
executing creduce on multiple files. Unfortunately, we were unable to reduce
to such a minimal example, yet. Nevertheless, this fix solved the problem in
protobuf.

To reproduce the error one must execute the analyzer with
-Xclang -analyzer-config -Xclang experimental-enable-naive-ctu-analysis=true -Xclang -analyzer-config -Xclang ctu-dir=/path/to/ctu_dir

Patch by Gabor Marton!

Differential Revision: https://reviews.llvm.org/D46019

llvm-svn: 331344