String used to do some operation by modifying itself
whereas OUString never does that and when a modificaiton is needed
it create a new copy.

so it is very easy when one convert String code to OUString code to
miss stuff like

sString.ToUpperCase()

which need to be converted into

sString = sString.toAsciiUpperCase()

and not
sString.toAsciiUpperCase()

This patch make the compiler generate a warning in that later _wrong_ case

Change-Id: I4a9c0b4c7d0b75ad8850ac23b86e8508a334f5fe
Signed-off-by: Stephan Bergmann <sbergman@redhat.com>