Clarify that o3tl::cmp_* is still needed for LLVM 12 libc++ for now

...which is apparently used by Android builds, so
<https://gerrit.libreoffice.org/c/core/+/158513> "Directly use std::cmp_*, drop
o3tl::cmp_*" cannot go in, yet

Change-Id: If8b3ef1e647b6e33c43d0fe5737daf63eda0b761
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/158524
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

include/o3tl/intcmp.hxx

@@ -20,7 +20,7 @@ namespace o3tl
 {
 // An approximation of the C++20 integer comparison functions
 // (<https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p0586r2.html> "Safe integral
-// comparisons"):
+// comparisons"), still missing from LLVM 12 libc++:
 #if defined __cpp_lib_integer_comparison_functions
 
 using std::cmp_equal;