Sanity: Don't spend too much time (or memory) on rediculously large enums

RemoteMirror gets called on lots of malformed type information,
due to memory corruption bugs or even clients that ask RemoteMirror
to decode a chunk of memory to test whether or not it might be
valid type data.  In any case, we need to be a little cautious here.

In this case, I've chosen to ignore any enum whose in-memory size
(according to the metadata) is over 1 MiB.  We can easily adjust
this limit up if experience shows there really are legitimate enums
this large in the wild.

Author: tbkka

stdlib/public/RemoteInspection/TypeLowering.cpp

@@ -1987,6 +1987,15 @@ class EnumTypeInfoBuilder {
     default: Kind = EnumKind::MultiPayloadEnum; break;
     }
 
+    // Sanity:  Ignore any enum that claims to have a size more than 1MiB
+    // This avoids allocating lots of memory for spare bit mask calculations
+    // when clients try to interpret random chunks of memory as type descriptions.
+    if (Size > (1024ULL * 1024)) {
+      unsigned Stride = ((Size + Alignment - 1) & ~(Alignment - 1));
+      return TC.makeTypeInfo<UnsupportedEnumTypeInfo>(
+	Size, Alignment, Stride, NumExtraInhabitants, BitwiseTakable, Kind, Cases);
+    }
+
     if (Cases.size() == 1) {
       if (EffectivePayloadCases == 0) {
         // Zero-sized enum with only one empty case