Sema: Don't reuse PreparedOverload from normal type checking in salvage()

We want to re-prepare our overloads, since now they will contain fixes.

lib/Sema/Constraint.cpp

@@ -20,6 +20,7 @@
 #include "swift/Basic/Compiler.h"
 #include "swift/Sema/Constraint.h"
 #include "swift/Sema/ConstraintSystem.h"
+#include "swift/Sema/PreparedOverload.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/raw_ostream.h"
 #include <algorithm>
@@ -1142,3 +1143,18 @@ void *Constraint::operator new(size_t bytes, ConstraintSystem& cs,
                                size_t alignment) {
   return ::operator new (bytes, cs, alignment);
 }
+
+// FIXME: Perhaps we should store the Constraint -> PreparedOverload mapping
+// in a SolverStep or something? Mutating Constraint feels wrong.
+
+void Constraint::setPreparedOverload(PreparedOverload *preparedOverload) {
+  ASSERT(Kind == ConstraintKind::BindOverload);
+
+  // We can only set a prepared overload at most once in normal
+  // type checking, and then once in salvage.
+  ASSERT(!Overload.Prepared ||
+         (!Overload.Prepared->wasForDiagnostics() &&
+          preparedOverload->wasForDiagnostics()));
+
+  Overload.Prepared = preparedOverload;
+}