We have many radars showing that stepping through C++ code can result in slow steps.

One of the things slowing us down is that ItaniumABILanguageRuntime class doesn't cache vtable to types in a map. This causes us, on every step, for every variable, to read the first pointer in a C++ type that could be dynamic and lookup the symbol, possibly in every symbol file (some symbols files on Darwin can end up having thousands of .o files when using DWARF in .o files, so thousands of .o files are searched each time). 

This fix caches lldb_private::Address (the resolved vtable symbol address in section + offset format) to TypeAndOrName instances inside the one ItaniumABILanguageRuntime in a process. This allows caching of dynamic types and stops us from always doing deep searches in each file.

<rdar://problem/18890778>

llvm-svn: 270488