[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/LanguageRuntime.cpp b/lldb/source/Target/LanguageRuntime.cpp
index 5c336ce..f60ea57 100644
--- a/lldb/source/Target/LanguageRuntime.cpp
+++ b/lldb/source/Target/LanguageRuntime.cpp
@@ -17,6 +17,8 @@
 using namespace lldb;
 using namespace lldb_private;
 
+char LanguageRuntime::ID = 0;
+
 ExceptionSearchFilter::ExceptionSearchFilter(const lldb::TargetSP &target_sp,
                                              lldb::LanguageType language,
                                              bool update_module_list)