[orc] Change LLJIT's dtor to a virtual dtor

[lxsameer]

Summary

Since LLLazyJIT inherits from LLJIT, the following is valid:

    
class A {
  std::unique_ptr<LLJIT> jit;
};
    
auto lazyJit = createLLLazyJIT(..);
A example(lazyJit);

But whet jit gets destroyed only the dtor of LLJIT will be called and
it will cause memory leaks on the members of LLLazyJIT.

With a virtual dtor for LLJIT, LLLazyJIT dtor will be called first.

[lxsameer]

here is an example of the memory leak:

=================================================================
==1915907==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 216 byte(s) in 1 object(s) allocated from:
    #0 0x7fb2d52da42d in operator new(unsigned long) /llvm-project/compiler-rt/lib/asan/asan_new_delete.cpp:95:3
    #1 0x7fb2d2c4cbd1 in llvm::orc::LLLazyJIT::LLLazyJIT(llvm::orc::LLLazyJITBuilderState&, llvm::Error&) (/build/src/libserene/libserene.so.0+0x1a00bd1)

Indirect leak of 176 byte(s) in 1 object(s) allocated from:
    #0 0x7fb2d52da42d in operator new(unsigned long) /llvm-project/compiler-rt/lib/asan/asan_new_delete.cpp:95:3
    #1 0x7fb2d2c414b4 in llvm::orc::createLocalLazyCallThroughManager(llvm::Triple const&, llvm::orc::ExecutionSession&, unsigned long) (/build/src/libserene/libserene.so.0+0x19f54b4)

Indirect leak of 152 byte(s) in 1 object(s) allocated from:
    #0 0x7fb2d52da42d in operator new(unsigned long) /llvm-project/compiler-rt/lib/asan/asan_new_delete.cpp:95:3
    #1 0x7fb2d2c32d93 in llvm::orc::LocalTrampolinePool<llvm::orc::OrcX86_64_SysV>::Create(llvm::unique_function<void (unsigned long, llvm::unique_function<void (unsigned long) const>) const>) IndirectionUtils.cpp

SUMMARY: AddressSanitizer: 544 byte(s) leaked in 3 allocation(s).

[lhames]

Looks good to me.

llvm/include/llvm/ExecutionEngine/Orc/LLJIT.h

public:
 
   protected:
     static void setInitTransform(LLJIT &J,
                                  IRTransformLayer::TransformFunction T);
   };
 
   /// Destruct this instance. If a multi-threaded instance, waits for all
   /// compile threads to complete.
-  ~LLJIT();
+  virtual ~LLJIT();
 
   /// Returns the ExecutionSession for this instance.
   ExecutionSession &getExecutionSession() { return *ES; }
 
   /// Returns a reference to the triple for this instance.
   const Triple &getTargetTriple() const { return TT; }
 
   /// Returns a reference to the DataLayout for this instance.