This change is necessary to differentiate between C++ const and non-const types in Swift.
For instance, `const int` and `int` would both be printed as `CInt` in Swift.
After this change, `const int` is stored as `CInt_const`
This is required to make the compiler distinguish between instantiations of `std::function`.
Previously, when generating a Swift type name for a `std::function` instantiation, we would always emit `_` as the template parameter. If someone referenced two different instantiations of `std::function` in a Swift module, they would get mangled with the same name, triggering linker errors later.
rdar://103979602
This makes sure we are printing more than one level of C++ template specializations when emitting a Swift struct name.
For instance, `std::__wrap_iter<char*>` and `std::__wrap_iter<const char*>` are currently imported with the same name in Swift. This means the mangled string will be the same for these specializations, despite them being distinct types. This causes mangling errors.
rdar://117485399
