Adapt the remaining OUString functions to std string_view

...for LIBO_INTERNAL_ONLY.  These had been missed by
1b43cceaea "Make many OUString functions take
std::u16string_view parameters" because they did not match the multi-overload
pattern that was addressed there, but they nevertheless benefit from being
changed just as well (witness e.g. the various resulting changes from copy() to
subView()).

This showed a conversion from OStringChar to std::string_view to be missing
(while the corresponding conversion form OUStringChar to std::u16string_view was
already present).

The improvement to loplugin:stringadd became necessary to fix

> [CPT] compilerplugins/clang/test/stringadd.cxx
> error: 'error' diagnostics expected but not seen:
>   File ~/lo/core/compilerplugins/clang/test/stringadd.cxx Line 43 (directive at ~/lo/core/compilerplugins/clang/test/stringadd.cxx:42): simplify by merging with the preceding assignment [loplugin:stringadd]
>   File ~/lo/core/compilerplugins/clang/test/stringadd.cxx Line 61 (directive at ~/lo/core/compilerplugins/clang/test/stringadd.cxx:60): simplify by merging with the preceding assignment [loplugin:stringadd]
> 2 errors generated.

Change-Id: Ie40de0616a66e60e289c1af0ca60aed6f9ecc279
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/107602
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

cpputools/source/unoexe/unoexe.cxx

@@ -18,6 +18,7 @@
  */
 
 #include <stdio.h>
+#include <string_view>
 
 #include <sal/main.h>
 #include <sal/log.hxx>
@@ -68,7 +69,7 @@ static void out( const char * pText )
         fprintf( stderr, "%s", pText );
 }
 
-static void out( const OUString & rText )
+static void out( std::u16string_view rText )
 {
     if (! s_quiet)
     {