[SIL] Switch "external" key path pattern components to SubstitutionMap.

Eliminates another source of SubstitutionList.

lib/SIL/SILPrinter.cpp

@@ -1065,6 +1065,18 @@ public:
     printDebugVar(ABI->getVarInfo());
   }
 
+  void printSubstitutions(SubstitutionMap Subs) {
+    if (Subs.empty())
+      return;
+
+    *this << '<';
+    interleave(Subs.getReplacementTypes(),
+               [&](Type type) { *this << type; },
+               [&] { *this << ", "; });
+    *this << '>';
+
+  }
+
   void printSubstitutions(SubstitutionList Subs) {
     if (Subs.empty())
       return;