...in code accidentally using auto like

> auto const aURL = uri->getUriReference() + "/"
>     + INetURLObject::encode(
>         m_sEmbeddedName, INetURLObject::PART_FPATH,
>         INetURLObject::EncodeMechanism::All);
>
> uno::Reference<uno::XInterface> xDataSource(xDatabaseContext->getByName(aURL), uno::UNO_QUERY);

in <https://gerrit.libreoffice.org/#/c/44569/1> "Properly construct
vnd.sun.star.pkg URL" did (causing hard to debug test failures there).

So make functions taking O[U]StringConcat take those by rvalue reference.
Unfortunately, that also needed adaption of various functions that just forward
their arguments.  And some code in sc/qa/unit/ucalc_formula.cxx used
CPPUNIT_ASSERT_EQUAL on OUStringConcat arguments in cases where that happened to
actually compile (because the structure of the two OUStringConcats was
identical), which needed adaption too (but which would arguably better use
CPPUNIT_ASSERT_EQUAL_MESSAGE, anyway).

Change-Id: I8994d932aaedb2a491c7c81c167e93379d4fb6e3
Reviewed-on: https://gerrit.libreoffice.org/44608Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>