Simplify a std::string_view construction

Simplify a std::string_view construction

...and combine three calls to SvStream::WriteOString into one.

Despite its name, SvStream::WriteOString takes a std::string_view, but which
cannot be constructed from a u8"..." char8_t string literal.  Besides the
OStringLiteral detour, other fixes could have been a
reinterpret_cast<char const *> (but which would have defeated constexpr'ness of
the std::string_view construction) or a plain "..." char string literal with
the UTF-8 encoding of \uFEFF spelled out as \0xEF\xBB\xBF.

Change-Id: I029f061134381a67516cbc4b0479942684c80c20
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/117461
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>