As a side-effect, this gets rid of some Clang
-fsanitize=implicit-signed-integer-truncation warnings.

The various external/harfbuzz/*.patch no longer applied and appear not to be
necessary any more.  (But a new external/harfbuzz/msvc.patch became necessary.)

<https://dev-www.libreoffice.org/src/harfbuzz-2.3.1.tar.bz2> was downloaded from
<https://www.freedesktop.org/software/harfbuzz/release/harfbuzz-2.3.1.tar.bz2>,
and HARFBUZZ_SHA256SUM in download.lst matches <https://www.freedesktop.org/
software/harfbuzz/release/harfbuzz-2.3.1.tar.bz2.sha256>.

Change-Id: Ic85acd14b4f488b3d88ce1bafc93be271928006e
Reviewed-on: https://gerrit.libreoffice.org/68731
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>