[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/CPPLanguageRuntime.cpp b/lldb/source/Target/CPPLanguageRuntime.cpp
index 70e5aee..836523a 100644
--- a/lldb/source/Target/CPPLanguageRuntime.cpp
+++ b/lldb/source/Target/CPPLanguageRuntime.cpp
@@ -35,6 +35,8 @@
 
 static ConstString g_this = ConstString("this");
 
+char CPPLanguageRuntime::ID = 0;
+
 // Destructor
 CPPLanguageRuntime::~CPPLanguageRuntime() {}