Implement C++14 numeric separators

C++14 allows the use of the apostrophe as a numeric separator; that
is, "23000" and "23'000" represent the same number.  This patch
implements this for gdb's C++ parser and the C++ name canonicalizer.

I did this unconditionally for all C variants because I think it's
unambiguous.

For the name canonicalizer, there's at least one compiler that can
emit constants with this form, see bug 30845.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=23457
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30845


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