[AST] Use SubstitutionMap, not SubstitutionList, in SILBoxType.

Eliminate the last place in the AST proper that stores
SubstitutionLists rather than SubstitutionMaps. Randomly fixes a
crasher, too.

include/swift/Serialization/ModuleFormat.h

@@ -55,7 +55,7 @@ const uint16_t VERSION_MAJOR = 0;
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
 /// Don't worry about adhering to the 80-column limit for this line.
-const uint16_t VERSION_MINOR = 413; // Last change: specialized conformance
+const uint16_t VERSION_MINOR = 414; // Last change: SIL box type substitutions
 
 using DeclIDField = BCFixed<31>;
 
@@ -800,8 +800,8 @@ namespace decls_block {
 
   using SILBoxTypeLayout = BCRecordLayout<
     SIL_BOX_TYPE,
-    SILLayoutIDField     // layout
-                         // trailing substitutions
+    SILLayoutIDField,     // layout
+    SubstitutionMapIDField // substitutions
   >;
 
   template <unsigned Code>