Use more *string_view

OUString::equalsIgnoreAsciiCaseAscii and compareToIgnoreAsciiCaseAscii
taking string_view are added as overloads, rather than replacements to
the variants taking const char*, because the latter have benefit when
the passed C-style string's length is unknown, because these variants
don't pre-calculate the length.

Change-Id: I2def689d7b7784212dc6cd6b8ae9ab1d42604079
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/150272
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>