Add new methods "subView" to O(U)String to return substring views
of the underlying data.
Add a clang plugin to warn when replacing existing calls to copy()
would be better to use subView().
Change-Id: I03a5732431ce60808946f2ce2c923b22845689ca
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/105420
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>