Fix UBSan report in StringMap implementation.

Use offsetof() instead of a member access within null pointer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223402 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/StringMap.h

@@ -182,10 +182,7 @@ class StringMapEntry : public StringMapEntryBase {
   /// GetStringMapEntryFromValue - Given a value that is known to be embedded
   /// into a StringMapEntry, return the StringMapEntry itself.
   static StringMapEntry &GetStringMapEntryFromValue(ValueTy &V) {
-    StringMapEntry *EPtr = 0;
-    char *Ptr = reinterpret_cast<char*>(&V) -
-                  (reinterpret_cast<char*>(&EPtr->second) -
-                   reinterpret_cast<char*>(EPtr));
+    char *Ptr = reinterpret_cast<char *>(&V) - offsetof(StringMapEntry, second);
     return *reinterpret_cast<StringMapEntry*>(Ptr);
   }
   static const StringMapEntry &GetStringMapEntryFromValue(const ValueTy &V) {