Perf: cache connection between const/non-const struct variants.

In one very large program, we were spending quite a bit of time repeatedly
getting const variants of StructTypes.  This speeds up the front-end by
about 40% for that test case.

(This is something of a band-aid, pending uniquing types.)

type.h

@@ -732,6 +732,8 @@ private:
     const Variability variability;
     const bool isConst;
     const SourcePos pos;
+
+    mutable const StructType *oppositeConstStructType;
 };