Fix the type-checker's recursive value type checking.

Track the types we've seen instead of the type declarations we've
passed through, which eliminates some holes relating to generic types.
Detect infinite expansions by imposing an arbitrary limit.

Fixes rdar://30355804

lib/Frontend/CompilerInvocation.cpp

@@ -957,6 +957,17 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     
     Opts.SolverMemoryThreshold = threshold;
   }
+
+  if (const Arg *A = Args.getLastArg(OPT_value_recursion_threshold)) {
+    unsigned threshold;
+    if (StringRef(A->getValue()).getAsInteger(10, threshold)) {
+      Diags.diagnose(SourceLoc(), diag::error_invalid_arg_value,
+                     A->getAsString(Args), A->getValue());
+      return true;
+    }
+
+    Opts.MaxCircularityDepth = threshold;
+  }
   
   for (const Arg *A : make_range(Args.filtered_begin(OPT_D),
                                  Args.filtered_end())) {