loplugin:useuniqueptr in LocaleDataWrapper

just use OUStringBuffer here, and consequently avoid re-implementing
such string-buffer handling code

Change-Id: I61e39dada6f46478b9d289f0310bb6846eb9868b
Reviewed-on: https://gerrit.libreoffice.org/62646
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

include/unotools/localedatawrapper.hxx

@@ -119,7 +119,7 @@ class UNOTOOLS_DLLPUBLIC LocaleDataWrapper
     void                getDefaultCalendarImpl();
     void                getSecondaryCalendarImpl();
 
-    sal_Unicode*        ImplAddFormatNum( sal_Unicode* pBuf,
+    void                ImplAddFormatNum( rtl::OUStringBuffer& rBuf,
                             sal_Int64 nNumber, sal_uInt16 nDecimals,
                             bool bUseThousandSep, bool bTrailingZeros ) const;