[ASTImporter] Add test for C++ casts and fix broken const_cast importing.

Summary:
The ASTImporter does currently not handle const_casts. This patch adds the
missing const_cast importer code and the test case that discovered this.

Reviewers: a.sidorin, a_sidorin

Reviewed By: a_sidorin

Subscribers: a_sidorin, martong, cfe-commits

Differential Revision: https://reviews.llvm.org/D50932

llvm-svn: 340182
diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
index c1136c9..8a8b321 100644
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -6897,6 +6897,10 @@
     return CXXReinterpretCastExpr::Create(
         Importer.getToContext(), ToType, VK, CK, ToOp, &BasePath,
         ToWritten, ToOperatorLoc, ToRParenLoc, ToAngleBrackets);
+  } else if (isa<CXXConstCastExpr>(E)) {
+    return CXXConstCastExpr::Create(Importer.getToContext(), ToType, VK, ToOp,
+                                    ToWritten, ToOperatorLoc, ToRParenLoc,
+                                    ToAngleBrackets);
   } else {
     return nullptr;
   }