Reinstate MSan suppression of PR24578.

[eugenis]

Summary

Revert "Rollback of commit "Repress sanitization on User dtor.""

There is no point in keeping an active MSan error in the codebase.
PR24578 tracks the actual UB in LLVM code; this change enables testing
of LLVM with MSAN + -fsanitize-memory-use-after-dtor.

This reverts commit 21c1bc46aee2b69c2c48db8e961f0ce8394f21e1.

[vitalybuka]

Isn't this going to break our msan bots?

llvm/include/llvm/Support/Compiler.h

 # define LLVM_PTR_SIZE sizeof(void *)
 #endif
 
 /// \macro LLVM_MEMORY_SANITIZER_BUILD
 /// Whether LLVM itself is built with MemorySanitizer instrumentation.
 #if __has_feature(memory_sanitizer)
 # define LLVM_MEMORY_SANITIZER_BUILD 1
 # include <sanitizer/msan_interface.h>
+# define LLVM_NO_SANITIZE_MEMORY_ATTRIBUTE __attribute__((no_sanitize_memory))
 #else
 # define LLVM_MEMORY_SANITIZER_BUILD 0
 # define __msan_allocated_memory(p, size)
 # define __msan_unpoison(p, size)
+# define LLVM_NO_SANITIZE_MEMORY_ATTRIBUTE
 #endif
 
 /// \macro LLVM_ADDRESS_SANITIZER_BUILD
 /// Whether LLVM itself is built with AddressSanitizer instrumentation.
 #if __has_feature(address_sanitizer) || defined(__SANITIZE_ADDRESS__)
 # define LLVM_ADDRESS_SANITIZER_BUILD 1
 # include <sanitizer/asan_interface.h>
 #else

llvm/lib/IR/Metadata.cpp

void *MDNode::operator new(size_t Size, unsigned NumOps) {
   OpSize = alignTo(OpSize, alignof(uint64_t));
   void *Ptr = reinterpret_cast<char *>(::operator new(OpSize + Size)) + OpSize;
   MDOperand *O = static_cast<MDOperand *>(Ptr);
   for (MDOperand *E = O - NumOps; O != E; --O)
     (void)new (O - 1) MDOperand;
   return Ptr;
 }
 
-void MDNode::operator delete(void *Mem) {
+// Repress memory sanitization, due to use-after-destroy by operator
+// delete. Bug report 24578 identifies this issue.
+LLVM_NO_SANITIZE_MEMORY_ATTRIBUTE void MDNode::operator delete(void *Mem) {
   MDNode *N = static_cast<MDNode *>(Mem);
   size_t OpSize = N->NumOperands * sizeof(MDOperand);
   OpSize = alignTo(OpSize, alignof(uint64_t));
 
   MDOperand *O = static_cast<MDOperand *>(Mem);
   for (MDOperand *E = O - N->NumOperands; O != E; --O)
     (O - 1)->~MDOperand();
   ::operator delete(reinterpret_cast<char *>(Mem) - OpSize);

llvm/lib/IR/User.cpp

void *User::operator new(size_t Size) {
   *HungOffOperandList = nullptr;
   return Obj;
 }
 
 //===----------------------------------------------------------------------===//
 //                         User operator delete Implementation
 //===----------------------------------------------------------------------===//
 
-void User::operator delete(void *Usr) {
+// Repress memory sanitization, due to use-after-destroy by operator
+// delete. Bug report 24578 identifies this issue.
+LLVM_NO_SANITIZE_MEMORY_ATTRIBUTE void User::operator delete(void *Usr) {
   // Hung off uses use a single Use* before the User, while other subclasses
   // use a Use[] allocated prior to the user.
   User *Obj = static_cast<User *>(Usr);
   if (Obj->HasHungOffUses) {
     assert(!Obj->HasDescriptor && "not supported!");
 
     Use **HungOffOperandList = static_cast<Use **>(Usr) - 1;
     // drop the hung off uses.