This patch allows qualifying methods with address spaces and extends some overloading rules to use those qualifiers.
The main use case is to prevent conversions to generic/default address space where it's requested by the programmer. More details can be found in:
http://lists.llvm.org/pipermail/cfe-dev/2018-December/060470.html
This patch doesn't enable the feature for C++ yet but it can easily be generalized if the overall flow is right.