cherry-pick "Adapt o3tl::span to removal of std::span::cbegin et al"

https://github.com/LibreOffice/core/commit/6fbfad6b

```
In file included from /builddir/build/BUILD/libreoffice-6.3.5/include/sfx2/dispatch.hxx:30:
/builddir/build/BUILD/libreoffice-6.3.5/include/o3tl/span.hxx:19:29: error: no member named 'span' in namespace 'std'
namespace o3tl { using std::span; }
                       ~~~~~^
1 warning generated.
In file included from /builddir/build/BUILD/libreoffice-6.3.5/sfx2/source/appl/appmisc.cxx:56:
/builddir/build/BUILD/libreoffice-6.3.5/include/sfx2/dispatch.hxx:146:46: error: no template named 'span' in namespace 'o3tl'
                                       o3tl::span<sal_uInt16 const> pSIDs = o3tl::span<sal_uInt16 const>());
                                       ~~~~~~^
/builddir/build/BUILD/libreoffice-6.3.5/include/sfx2/dispatch.hxx:146:83: error: no member named 'span' in namespace 'o3tl'
                                       o3tl::span<sal_uInt16 const> pSIDs = o3tl::span<sal_uInt16 const>());
                                                                            ~~~~~~^
/builddir/build/BUILD/libreoffice-6.3.5/include/sfx2/dispatch.hxx:146:88: error: unexpected type name 'sal_uInt16': expected expression
                                       o3tl::span<sal_uInt16 const> pSIDs = o3tl::span<sal_uInt16 const>());
                                                                                       ^
1 warning and 4 errors generated.
```

6fbfad6b00e8c35346ee59cd32a0d7ccc0d8c19c.patch

@@ -0,0 +1,61 @@
+From 6fbfad6b00e8c35346ee59cd32a0d7ccc0d8c19c Mon Sep 17 00:00:00 2001
+From: Stephan Bergmann <sbergman@redhat.com>
+Date: Sat, 22 Feb 2020 17:29:15 +0100
+Subject: [PATCH] Adapt o3tl::span to removal of std::span::cbegin et al
+
+<https://cplusplus.github.io/LWG/issue3320> "span::cbegin/cend methods produce
+different results than std::[ranges::]cbegin/cend", as implemented now in
+<https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;a=commit;
+h=247f410b83797a1840573840cc2a539ef9d7f96b> "libstdc++: Remove std::span::cbegin
+and std::span::cend (LWG 3320)".
+
+Turns out we only used the removed member functions in o3tl/qa/test-span.cxx.
+
+Change-Id: I6c73797594b4e0e753a88840033d54961e271df5
+Reviewed-on: https://gerrit.libreoffice.org/c/core/+/89261
+Tested-by: Jenkins
+Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
+---
+ include/o3tl/span.hxx | 7 -------
+ o3tl/qa/test-span.cxx | 6 ------
+ 2 files changed, 13 deletions(-)
+
+--- libreoffice-6.3.5.2/include/o3tl/span.hxx.orig	2020-04-04 16:27:13.798399505 +0300
++++ libreoffice-6.3.5.2/include/o3tl/span.hxx	2020-04-04 16:28:05.080888607 +0300
+@@ -60,18 +60,11 @@
+     constexpr iterator begin() const noexcept { return data_; }
+     constexpr iterator end() const noexcept { return begin() + size(); }
+ 
+-    constexpr const_iterator cbegin() const noexcept { return begin(); }
+-    constexpr const_iterator cend() const noexcept { return end(); }
+-
+     reverse_iterator rbegin() const noexcept
+     { return reverse_iterator(end()); }
+     reverse_iterator rend() const noexcept
+     { return reverse_iterator(begin()); }
+ 
+-    constexpr const_reverse_iterator crbegin() const noexcept
+-    { return rbegin(); }
+-    constexpr const_reverse_iterator crend() const noexcept { return rend(); }
+-
+     constexpr index_type size() const noexcept { return size_; }
+ 
+     constexpr reference operator [](index_type pos) const {
+--- libreoffice-6.3.5.2/o3tl/qa/test-span.cxx.orig	2020-04-04 16:27:13.802399388 +0300
++++ libreoffice-6.3.5.2/o3tl/qa/test-span.cxx	2020-04-04 16:30:35.256474807 +0300
+@@ -33,15 +33,9 @@
+         CPPUNIT_ASSERT_EQUAL(1, *v.begin());
+         CPPUNIT_ASSERT_EQUAL(
+             o3tl::span<int>::difference_type(3), v.end() - v.begin());
+-        CPPUNIT_ASSERT_EQUAL(1, *v.cbegin());
+-        CPPUNIT_ASSERT_EQUAL(
+-            o3tl::span<int>::difference_type(3), v.cend() - v.cbegin());
+         CPPUNIT_ASSERT_EQUAL(3, *v.rbegin());
+         CPPUNIT_ASSERT_EQUAL(
+             o3tl::span<int>::difference_type(3), v.rend() - v.rbegin());
+-        CPPUNIT_ASSERT_EQUAL(3, *v.crbegin());
+-        CPPUNIT_ASSERT_EQUAL(
+-            o3tl::span<int>::difference_type(3), v.crend() - v.crbegin());
+         CPPUNIT_ASSERT_EQUAL(o3tl::span<int>::index_type(3), v.size());
+         CPPUNIT_ASSERT(!v.empty());
+         CPPUNIT_ASSERT_EQUAL(2, v[1]);

libreoffice.spec

@@ -84,6 +84,8 @@ Source1000:	libreoffice.rpmlintrc
 Patch0:		libreoffice-5.4.0.1-disable-test-languagetag.patch
 Patch1:		libreoffice-4.2.5-icu-49.patch
 Patch2:		libreoffice-6.3.5.2-kde4-detection.patch
+# https://github.com/LibreOffice/core/commit/6fbfad6b
+Patch3:		6fbfad6b00e8c35346ee59cd32a0d7ccc0d8c19c.patch
 
 # ROSA vendor patch
 Patch100:	libreoffice-4.1-vendor.patch