[Mirror] Don't destroy and reconstitute enums when reflecting them.

It's possible to get multiple threads reading from the same enum at the same time, and the result can be bad data extracted or even permanent corruption of the value in memory. Instead, copy the enum value, then project the data from the copy.

rdar://problem/59493486

Author: mikeash

stdlib/public/runtime/ReflectionMirror.mm

@@ -488,14 +488,21 @@ AnyReturn subscript(intptr_t i, const char **outName,
 
     auto *caseName = getInfo(&tag, &payloadType, &indirect);
 
+    // Copy the enum itself so that we can project the data without destroying
+    // the original.
+    Any enumCopy;
+    auto *enumCopyContainer
+      = type->allocateBoxForExistentialIn(&enumCopy.Buffer);
+    type->vw_initializeWithCopy(enumCopyContainer,
+                                const_cast<OpaqueValue *>(value));
+
     // Copy the enum payload into a box
     const Metadata *boxType = (indirect ? &METADATA_SYM(Bo).base : payloadType);
     BoxPair pair = swift_allocBox(boxType);
-
-    type->vw_destructiveProjectEnumData(const_cast<OpaqueValue *>(value));
-    boxType->vw_initializeWithCopy(pair.buffer, const_cast<OpaqueValue *>(value));
-    type->vw_destructiveInjectEnumTag(const_cast<OpaqueValue *>(value), tag);
-
+    type->vw_destructiveProjectEnumData(enumCopyContainer);
+    boxType->vw_initializeWithTake(pair.buffer, enumCopyContainer);
+    type->deallocateBoxForExistentialIn(&enumCopy.Buffer);
+    
     value = pair.buffer;
 
     // If the payload is indirect, we need to jump through the box to get it.