[LoopNest] Fix Wdeprecated-copy warnings

[Whitney]

Summary

error: definition of implicit copy constructor for 'LoopNest' is deprecated because it has a user-declared copy assignment operator [-Werror,-Wdeprecated-copy]
  LoopNest &operator=(const LoopNest &) = delete;

[Meinersbur]

LGTM

[eugenis]

It appears that this change broke the sanitizer-windows bot:
https://lab.llvm.org/buildbot/#/builders/127/builds/12064

[Whitney]

In D103752#2806386, @eugenis wrote:

It appears that this change broke the sanitizer-windows bot:
https://lab.llvm.org/buildbot/#/builders/127/builds/12064

I am having problem reproducing it, and have emailed @rnk and @gkistanova to get help.

[eugenis]

Try reverting it in the meantime?

[eugenis]

The buildbot went green before your revert has landed.
No idea what's going on, but this change does not seem to be the cause of it.

[rnk]

In D103752#2806569, @eugenis wrote:

The buildbot went green before your revert has landed.
No idea what's going on, but this change does not seem to be the cause of it.

I saw the PDB related error and forced a clean build. That turned the bot green again.

llvm/include/llvm/Analysis/LoopNestAnalysis.h

 
 /// This class represents a loop nest and can be used to query its properties.
 class LoopNest {
 public:
   /// Construct a loop nest rooted by loop \p Root.
   LoopNest(Loop &Root, ScalarEvolution &SE);
 
   LoopNest() = delete;
-  LoopNest &operator=(const LoopNest &) = delete;
 
   /// Construct a LoopNest object.
   static std::unique_ptr<LoopNest> getLoopNest(Loop &Root, ScalarEvolution &SE);
 
   /// Return true if the given loops \p OuterLoop and \p InnerLoop are
   /// perfectly nested with respect to each other, and false otherwise.
   /// Example:
   /// \code