xgettext: Add support for gettext-like functions with wide string argument.

xgettext: Add support for gettext-like functions with wide string argument.

Reported by Érico Nogueira <ericonr@disroot.org> in
<https://lists.gnu.org/archive/html/bug-gettext/2020-11/msg00013.html>.

* gettext-tools/src/x-c.c (phase5_get): Recognize also the string-literal
prefixes 'u', 'U', 'L'. Consider all types of C++ raw strings as relevant.
* gettext-tools/tests/xgettext-c-5: Add more test cases.
* gettext-tools/tests/xgettext-c-c++-1: Likewise.
* NEWS: Mention it.