[LanguageRuntime] Introduce LLVM-style casts

Summary:
Using llvm-style rtti gives us stronger guarantees around casting
LanguageRuntimes.

As discussed in D62755

Differential Revision: https://reviews.llvm.org/D62934

llvm-svn: 362884
diff --git a/lldb/source/Target/ObjCLanguageRuntime.cpp b/lldb/source/Target/ObjCLanguageRuntime.cpp
index 106ea07..3084b70e 100644
--- a/lldb/source/Target/ObjCLanguageRuntime.cpp
+++ b/lldb/source/Target/ObjCLanguageRuntime.cpp
@@ -28,6 +28,8 @@
 using namespace lldb;
 using namespace lldb_private;
 
+char ObjCLanguageRuntime::ID = 0;
+
 // Destructor
 ObjCLanguageRuntime::~ObjCLanguageRuntime() {}