In order to support the Microsoft ABI alongside the Itanium one in the
same process from different DLLs, this moves the Itanium ABI runtime
plugin to the C++ language runtime (see
https://github.com/llvm/llvm-project/pull/168941#discussion_r2547684264).
Before this PR, the C++ language runtime wasn't a plugin. Instead, its
functionality was provided by the Itanium ABI plugin.
All Itanium specific methods are moved to a new class
`ItaniumABIRuntime`. This includes resolving the dynamic type, setting
exception filters, and getting the exception object.
The other methods were added to `CPPLanguageRuntime`.
`language cplusplus demangle` moved to `CommandObjectCPlusPlus`.
The Clang REPL depended on the C++ runtime. Now that it's a plugin, this
failed the layering check. Since the REPL doesn't use the C++ runtime, I
removed the dependency.
