...with numeric values that are not meant to be characters.  Inspired by
<https://gerrit.libreoffice.org/#/c/16339/> "OUString: add constructor from
single Unicode code point" and ecaf1cb3 "seems
to be what was meant."  Found e46e8d19 "Shall
this produce a string representation of a number?" and
f2a0e403 "Don't truncate a UTF-32 code point to
a UTF-16 code unit."

(LIBO_INTERNAL_ONLY mostly because it uses a C++11 delegating ctor for
simplicity, but also because it requires mild modifications to client code, see
additional changes in this commit and some other recent commits.)

Change-Id: I8a156f01d1b772c6052d7afaa088ae13ed234e2b