[PassManager] Make PassManagerBuilder::addExtension take an std::function, rather than a function pointer.

[jlebar]

Summary

This gives callers flexibility to pass lambdas with captures, which lets
callers avoid the C-style void*-ptr closure style. (Currently, callers
in clang store state in the PassManagerBuilderBase arg.)

No functional change, and the new API is backwards-compatible.

[mehdi_amini]

LGTM.
Adding stuff in PassManagerBuilderBase for the purpose of extension was... unfortunate.

include/llvm/Transforms/IPO/PassManagerBuilder.h

 ///   ...
 ///   Builder.addExtension(PassManagerBuilder::EP_LoopOptimizerEnd,
 ///                        addMyLoopPass);
 ///   ...
 class PassManagerBuilder {
 public:
   /// Extensions are passed the builder itself (so they can see how it is
   /// configured) as well as the pass manager to add stuff to.
-  typedef void (*ExtensionFn)(const PassManagerBuilder &Builder,
-                              legacy::PassManagerBase &PM);
+  typedef std::function<void(const PassManagerBuilder &Builder,
+                             legacy::PassManagerBase &PM)>
+      ExtensionFn;
   enum ExtensionPointTy {
     /// EP_EarlyAsPossible - This extension point allows adding passes before
     /// any other transformations, allowing them to see the code as it is coming
     /// out of the frontend.
     EP_EarlyAsPossible,
 
     /// EP_ModuleOptimizerEarly - This extension point allows adding passes
     /// just before the main module-level optimization passes.
public:
 
   /// Profile data file name that the instrumentation will be written to.
   std::string PGOInstrGen;
   /// Path of the profile data file.
   std::string PGOInstrUse;
 
 private:
   /// ExtensionList - This is list of all of the extensions that are registered.
-  std::vector<std::pair<ExtensionPointTy, ExtensionFn> > Extensions;
+  std::vector<std::pair<ExtensionPointTy, ExtensionFn>> Extensions;
 
 public:
   PassManagerBuilder();
   ~PassManagerBuilder();
   /// Adds an extension that will be used by all PassManagerBuilder instances.
   /// This is intended to be used by plugins, to register a set of
   /// optimisations to run automatically.
   static void addGlobalExtension(ExtensionPointTy Ty, ExtensionFn Fn);

 
 /// Registers a function for adding a standard set of passes.  This should be
 /// used by optimizer plugins to allow all front ends to transparently use
 /// them.  Create a static instance of this class in your plugin, providing a
 /// private function that the PassManagerBuilder can use to add your passes.
 struct RegisterStandardPasses {
   RegisterStandardPasses(PassManagerBuilder::ExtensionPointTy Ty,
                          PassManagerBuilder::ExtensionFn Fn) {
-    PassManagerBuilder::addGlobalExtension(Ty, Fn);
+    PassManagerBuilder::addGlobalExtension(Ty, std::move(Fn));
   }
 };
 
 } // end namespace llvm
 #endif

lib/Transforms/IPO/PassManagerBuilder.cpp

 
 /// Set of global extensions, automatically added as part of the standard set.
 static ManagedStatic<SmallVector<std::pair<PassManagerBuilder::ExtensionPointTy,
    PassManagerBuilder::ExtensionFn>, 8> > GlobalExtensions;
 
 void PassManagerBuilder::addGlobalExtension(
     PassManagerBuilder::ExtensionPointTy Ty,
     PassManagerBuilder::ExtensionFn Fn) {
-  GlobalExtensions->push_back(std::make_pair(Ty, Fn));
+  GlobalExtensions->push_back(std::make_pair(Ty, std::move(Fn)));
 }
 
 void PassManagerBuilder::addExtension(ExtensionPointTy Ty, ExtensionFn Fn) {
-  Extensions.push_back(std::make_pair(Ty, Fn));
+  Extensions.push_back(std::make_pair(Ty, std::move(Fn)));
 }
 
 void PassManagerBuilder::addExtensionsToPM(ExtensionPointTy ETy,
                                            legacy::PassManagerBase &PM) const {
   for (unsigned i = 0, e = GlobalExtensions->size(); i != e; ++i)
     if ((*GlobalExtensions)[i].first == ETy)
       (*GlobalExtensions)[i].second(*this, PM);
   for (unsigned i = 0, e = Extensions.size(); i != e; ++i)