Fix C++ name canonicalizations of character literals

The names "void C<(char)1>::m()" and "void C<'\001'>::m()" should
canonicalize to the same string, but currently they do not -- the
former remains unchanged and the latter is transformed to
"void C<(char)'\001'>::m()".

This patch fixes the bug and also adds some unit tests.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=16843


Approved-By: John Baldwin <jhb@FreeBSD.org>