Basic: avoid an assertion on Windows

The order of evaluation of the subscript operator would increase the
count of map, resulting in the decoration to be off by one.  This
enables building the swift standard library on Windows again.

Author: compnerd

include/swift/Basic/Mangler.h

@@ -124,12 +124,18 @@ class Mangler {
   void appendIdentifier(StringRef ident);
 
   void addSubstitution(const void *ptr) {
-    if (UseSubstitutions)
-      Substitutions[ptr] = Substitutions.size() + StringSubstitutions.size();
+    if (!UseSubstitutions)
+      return;
+
+    auto value = Substitutions.size() + StringSubstitutions.size();
+    Substitutions[ptr] = value;
   }
   void addSubstitution(StringRef Str) {
-    if (UseSubstitutions)
-      StringSubstitutions[Str] = Substitutions.size() + StringSubstitutions.size();
+    if (!UseSubstitutions)
+      return;
+
+    auto value = Substitutions.size() + StringSubstitutions.size();
+    StringSubstitutions[Str] = value;
   }
 
   bool tryMangleSubstitution(const void *ptr);