Work around clang-cl issue with MSVC's safeint.h for now

At least some C:\Program Files (x86)\Windows Kits\10\Include\10.0.10240.0\ucrt\
safeint_internal.h, in the specialization of LargeIntRegMultiply::RegMultiply
for signed __int64, contains an (unnecessary) C-style cast from l-value unsigned
__int64 tmp

  RegMultiply( ..., ..., (unsigned _int64)tmp ) == SafeIntNoError )

which clang-cl correctly identifies as producing a prvalue (so cannot bind to
the third 'unsigned __int64& ret' parameter of that RegMultiply call), while
MSVC apparently has a non-standard mis-feature of treating such a "self-cast" as
an lvalue.

Until clang-cl learns about that mis-feature too, use the fallback
implementation of o3tl/safeint.hxx when compiling with clang-cl.

Change-Id: I7a454134589d958ad7ba845c37584b3aa7b94926
Reviewed-on: https://gerrit.libreoffice.org/36066Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

include/o3tl/safeint.hxx

@@ -11,7 +11,7 @@
 #define INCLUDED_O3TL_SAFEINT_HXX
 
 #include <limits>
-#if defined(_MSC_VER)
+#if defined(_MSC_VER) && !defined __clang__
 #include <safeint.h>
 #else
 #ifndef __has_builtin
@@ -22,7 +22,7 @@
 namespace o3tl
 {
 
-#if defined(_MSC_VER)
+#if defined(_MSC_VER) && !defined __clang__
 
 template<typename T> inline bool checked_multiply(T a, T b, T& result)
 {