Eliminate the ItaniumVTableContext object from CodeGenVTables

Now CodeGenVTables has only one VTableContext object, which is either
Itanium or Microsoft.

Fixes a FIXME with no functionality change intended.

Ideally we could avoid the downcasts by pushing the things that
reference the Itanium vtable context into ItaniumCXXABI.cpp, but we're
not there yet.