Add -warn-long-expression-type-checking=<limit> frontend option.

Generates a warning for any expression that takes longer than <limit>
milliseconds to type check. This compliments the existing
-warn-long-function-body=<limit> option.

lib/Frontend/CompilerInvocation.cpp

@@ -216,6 +216,16 @@ static bool ParseFrontendArgs(FrontendOptions &Opts, ArgList &Args,
     }
   }
 
+  if (const Arg *A = Args.getLastArg(OPT_warn_long_expression_type_checking)) {
+    unsigned attempt;
+    if (StringRef(A->getValue()).getAsInteger(10, attempt)) {
+      Diags.diagnose(SourceLoc(), diag::error_invalid_arg_value,
+                     A->getAsString(Args), A->getValue());
+    } else {
+      Opts.WarnLongExpressionTypeChecking = attempt;
+    }
+  }
+
   Opts.PlaygroundTransform |= Args.hasArg(OPT_playground);
   if (Args.hasArg(OPT_disable_playground_transform))
     Opts.PlaygroundTransform = false;