WG14: Trimming whitespace before line splicing

[peter-b]

P2223 looks like it's on track for C++23. With regards to C compatibility, P2223 says:

In effect, the ”implementation defined mapping: in phase one makes the content of the program implementation defined, and it is a valid behavior in C and C++ alike to trim or not trailing whitespaces. The proposed change only requires that all C++ compilers do this trimming, and therefore doesn’t affect C compatibility.

This opens the possibility that C implementations that also implement C++ might change their behaviour in order to make sure that compiling the same header in C and C++ mode doesn't give different results. We should see if WG14 want to make similar changes to the C standard.