Correctly calculate extra inhabitants for no-payload enums (#28830)

In particular, this fixes the size calculation for nested enums,
specifically enums within Optionals.  Without this, the
reflection library computes `v` below as requiring two bytes
instead of one.

```
enum E {
case a
case b
}

let v = Optional<E>
```

This also adds a number of test cases for enums alone and
wrapped in optionals, including:
* Zero-case enums are allocated zero size and have zero extra inhabitants
* Zero-case enums in optionals also get zero size
* One-case no-payload enums are allocated zero size and have zero extra inhabitants
* One-case no-payload enums in optionals get one byte allocated and have zero extra inhabitants
* 254-case enums have only two extra inhabitants, so putting them in thrice-nested optionals requires an extra byte
* Various cases where each nested optional gets an extra byte

Resolves rdar://31154770

Author: tbkka

stdlib/public/Reflection/TypeLowering.cpp

@@ -1007,10 +1007,22 @@ class EnumTypeInfoBuilder {
     // NoPayloadEnumImplStrategy
     if (PayloadCases.empty()) {
       Kind = RecordKind::NoPayloadEnum;
-      Size += getEnumTagCounts(/*size=*/0,
-                               NoPayloadCases,
-                               /*payloadCases=*/0).numTagBytes;
-
+      switch (NoPayloadCases) {
+      case 0:
+      case 1: // Zero or one tag has size = 0, extra_inhab = 0
+        NumExtraInhabitants = 0;
+        break;
+      default: { // 2 or more tags
+        auto tagCounts = getEnumTagCounts(/*size=*/0,
+                                          NoPayloadCases,
+                                          /*payloadCases=*/0);
+        Size += tagCounts.numTagBytes;
+        NumExtraInhabitants =
+          (1 << (tagCounts.numTagBytes * 8)) - tagCounts.numTags;
+        NumExtraInhabitants = std::min(NumExtraInhabitants,
+                     unsigned(ValueWitnessFlags::MaxNumExtraInhabitants));
+      }
+      }
     // SinglePayloadEnumImplStrategy
     } else if (PayloadCases.size() == 1) {
       auto *CaseTR = getCaseTypeRef(PayloadCases[0]);

test/Reflection/typeref_lowering.swift

@@ -1039,9 +1039,9 @@
 // CHECK-64-NEXT:   (field name=empty offset=0
 // CHECK-64-NEXT:     (no_payload_enum size=0 alignment=1 stride=1 num_extra_inhabitants=0 bitwise_takable=1))
 // CHECK-64-NEXT:   (field name=noPayload offset=0
-// CHECK-64-NEXT:     (no_payload_enum size=1 alignment=1 stride=1 num_extra_inhabitants=0 bitwise_takable=1))
+// CHECK-64-NEXT:     (no_payload_enum size=1 alignment=1 stride=1 num_extra_inhabitants=252 bitwise_takable=1))
 // CHECK-64-NEXT:   (field name=sillyNoPayload offset=1
-// CHECK-64-NEXT:     (no_payload_enum size=1 alignment=1 stride=1 num_extra_inhabitants=0 bitwise_takable=1))
+// CHECK-64-NEXT:     (no_payload_enum size=1 alignment=1 stride=1 num_extra_inhabitants=252 bitwise_takable=1))
 // CHECK-64-NEXT:   (field name=singleton offset=8
 // CHECK-64-NEXT:     (reference kind=strong refcounting=native))
 // CHECK-64-NEXT:   (field name=singlePayload offset=16