[lldb][modern-type-lookup] Fix crash when activating modern-type-lookup on Linux

There is no ClangModulesDeclVendor on Linux so that cast is triggering an assert.
Let's just remove it as it just casts the type to itself.

llvm-svn: 372974

lldb/source/Plugins/ExpressionParser/Clang/ClangASTSource.cpp

@@ -108,8 +108,7 @@ void ClangASTSource::InstallASTContext(clang::ASTContext &ast_context,
     } while (false);
 
     do {
-      auto *modules_decl_vendor = llvm::cast<ClangModulesDeclVendor>(
-          m_target->GetClangModulesDeclVendor());
+      auto *modules_decl_vendor = m_target->GetClangModulesDeclVendor();
 
       if (!modules_decl_vendor)
         break;