[PATCH, C++-1z] Implement N4197 - Adding u8 character literals

[Ed Smith-Rowland <3dw4rd at verizon dot net>]

I apologize to all who got this earlier.
I forgot to send to the list.

This patch adds UTF-8 character literals to C++:

    auto c = u8'c';

This is mostly a straight forward preprocessor exercise.
I only allow these characters in -std=c++1z and -std=gnu++1z.
I figure we can expand that easily with or without some pedwarn.

In c-ada-specs.c/print_ada_macros() I just write these as a char 
constant rather than spelling the token.
We could do the latter.  You'd see the "u8" then I think.  I couldn't 
find in the Ada test suite where this was exercised.

That's all that seems controversial to me.

Built and tested clean on x86_64-linux.

Ed

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