[PM] Make AnalysisManager::registerPass take its parameter by universal reference.

[jlebar]

Summary

Previously, we were forcing a copy if you passed an lvalue argument; now
we'll take it by reference.

[silvas]

This is fine, but it doesn't really matter; this is only called like once for an entire compilation (it's basically a hack to set a pipeline-global closure for a particular analysis; only AA uses it now)

[jlebar]

Thank you for the reviews on this patch queue, Sean!

llvm/include/llvm/IR/PassManager.h

public:
   ///
   /// While this returns whether or not the pass type was already registered,
   /// there in't an independent way to query that as that would be prone to
   /// risky use when *querying* the analysis manager. Instead, the only
   /// supported use case is avoiding duplicate registry of an analysis. This
   /// interface also lends itself to minimizing the number of times we have to
   /// do lookups for analyses or construct complex passes only to throw them
   /// away.
-  template <typename PassBuilderT> bool registerPass(PassBuilderT PassBuilder) {
+  template <typename PassBuilderT>
+  bool registerPass(PassBuilderT &&PassBuilder) {
     typedef decltype(PassBuilder()) PassT;
     typedef detail::AnalysisPassModel<IRUnitT, PassT, PreservedAnalyses,
                                       Invalidator, ExtraArgTs...>
         PassModelT;
 
     auto &PassPtr = AnalysisPasses[PassT::ID()];
     if (PassPtr)
       // Already registered this pass type!