clang/Modules: Bring back optimization lost in 31e14f4

31e14f4 accidentally dropped caching of
failed module loads.  This brings it back by making
ModuleMap::getCachedModuleLoad return an Optional.

Author: dexonsmith

clang/include/clang/Lex/ModuleMap.h

@@ -707,8 +707,11 @@ class ModuleMap {
   }
 
   /// Return a cached module load.
-  Module *getCachedModuleLoad(const IdentifierInfo &II) {
-    return CachedModuleLoads.lookup(&II);
+  llvm::Optional<Module *> getCachedModuleLoad(const IdentifierInfo &II) {
+    auto I = CachedModuleLoads.find(&II);
+    if (I == CachedModuleLoads.end())
+      return None;
+    return I->second;
   }
 };
 

clang/lib/Frontend/CompilerInstance.cpp

@@ -1652,10 +1652,11 @@ CompilerInstance::loadModule(SourceLocation ImportLoc,
   }
 
   // If we don't already have information on this module, load the module now.
+  Module *Module = nullptr;
   ModuleMap &MM = getPreprocessor().getHeaderSearchInfo().getModuleMap();
-  clang::Module *Module = MM.getCachedModuleLoad(*Path[0].first);
-  if (Module) {
-    // Nothing to do here, we found it.
+  if (auto MaybeModule = MM.getCachedModuleLoad(*Path[0].first)) {
+    // Use the cached result, which may be nullptr.
+    Module = *MaybeModule;
   } else if (ModuleName == getLangOpts().CurrentModule) {
     // This is the module we're building.
     Module = PP->getHeaderSearchInfo().lookupModule(