CrossModuleOptimization: fix a bug in cycle-detection

The check to avoid infinite recursion in case of call graph cycles didn't work correctly.
It didn't result in crashes, because the function also has an additional max-depth check, but it could lead to exponential complexity in some cases.

Unfortunately I don't have a test case for this fix.

lib/SILOptimizer/IPO/CrossModuleOptimization.cpp

@@ -185,6 +185,10 @@ bool CrossModuleOptimization::canSerializeFunction(
   if (iter != canSerializeFlags.end())
     return iter->second;
 
+  // Temporarily set the flag to false (to avoid infinite recursion) until we set
+  // it to true at the end of this function.
+  canSerializeFlags[function] = false;
+
   if (DeclContext *funcCtxt = function->getDeclContext()) {
     if (!canUseFromInline(funcCtxt))
       return false;