RFE: -fconcepts support for libstdc++

[Avi Kivity]

C++ error messages are notoriously cryptic, but it's much worse when it 
comes from someone else code.


In our code, we have a GCC6_CONCEPT(x) macro that wraps concept 
declarations and is defined as x when -fconcepts is enabled. It would be 
wonderful if something similar could be done to libstdc++ when 
-fconcepts is enabled.


Now, the concept implementation in gcc may not match the draft standard 
exactly, but gcc and libstdc++ are developed in lockstep so when the 
compiler changes the library can change with it. We won't be able to 
rely on concepts for disambiguating between overloads, but they'll work 
well for improving the quality of diagnostics.