\@@_declare_shapes_smcaps:nn setup a slsc -> itsc substitution

If a slanted font is not provided, fontspec setup automatic `sl -> it`
substitutions in `\@@_declare_shape_slanted:nn`. But if a small caps
font was provided, an `itsc` shape is defined but there is no
corresponding `slsc -> itsc` substitution.

Fix this by adapting the code of `\@@_declare_shape_slanted:nn` into
`\@@_declare_shapes_smcaps:nn`.

fontspec-code-internal.dtx

@@ -815,13 +815,24 @@
 % \end{macro}
 %
 % \begin{macro}{\@@_declare_shapes_smcaps:nn}
+% We set up a smallcaps font if it has been provided. If the current shape
+% is `it', we also set up a `slsc -> itsc` substitution, like in \verb|\@@_declare_shape_slanted:nn|.
 %    \begin{macrocode}
 \cs_new:Nn \@@_declare_shapes_smcaps:nn
   {
     \tl_if_empty:NF \l_@@_nfss_sc_tl
      {
       \@@_DeclareFontShape:xxxxxx {\g_@@_nfss_enc_tl} {\g_@@_nfss_family_tl} {#1}
         { \@@_combo_sc_shape:n {#2} } {\l_@@_nfss_sc_tl} {\l_@@_postadjust_tl}
+      \bool_if:nT
+        {
+            \str_if_eq_p:ee {#2} {\itdefault}  &&
+          !(\str_if_eq_p:ee {\itscdefault} {\slscdefault})
+        }
+        {
+          \@@_DeclareFontShape:xxxxxx {\g_@@_nfss_enc_tl}{\g_@@_nfss_family_tl}{#1}{\slscdefault}
+            {<->ssub*\g_@@_nfss_family_tl/#1/\itscdefault}{\l_@@_postadjust_tl}
+        }
      }
   }
 %    \end{macrocode}