Ensure stdlib parses again after #79186.

stdlib/public/core/CollectionAlgorithms.swift

@@ -441,6 +441,7 @@ extension MutableCollection where Self: BidirectionalCollection {
       swapAt(lo, hi)
       formIndex(after: &lo)
     }
+    fatalError()
   }
 }
 

stdlib/public/core/Flatten.swift

@@ -85,6 +85,7 @@ extension FlattenSequence.Iterator: IteratorProtocol {
       _inner = s!.makeIterator()
     }
     while true
+    fatalError()
   }
 }
 

stdlib/public/core/HashTable.swift

@@ -386,6 +386,7 @@ extension _HashTable {
         return Bucket(word: word, bit: bit)
       }
     }
+    fatalError()
   }
 
   @inlinable
@@ -413,6 +414,7 @@ extension _HashTable {
         return Bucket(word: word, bit: bit)
       }
     }
+    fatalError()
   }
 }
 

stdlib/public/core/Join.swift

@@ -125,6 +125,7 @@ extension JoinedSequence.Iterator: IteratorProtocol {
         return nil
       }
     }
+    fatalError()
   }
 }
 

stdlib/public/core/KeyPath.swift

@@ -192,6 +192,7 @@ public class AnyKeyPath: _AppendKeyPath {
 
         if optNextType == nil { return .some(offset) }
       }
+      fatalError()
     }
     #else
     // compiler optimizes _storedInlineOffset into a direct offset computation,
@@ -270,6 +271,7 @@ extension AnyKeyPath: Hashable {
             return true
           }
         }
+        fatalError()
       }
     }
   }
@@ -431,6 +433,7 @@ public class KeyPath<Root, Value>: PartialKeyPath<Root> {
             }
           }
         }
+        fatalError()
       }
     }
   }

stdlib/public/core/SequenceAlgorithms.swift

@@ -332,6 +332,7 @@ extension Sequence {
       case (nil, nil):           return true
       }
     }
+    fatalError()
   }
 }
 
@@ -425,6 +426,7 @@ extension Sequence {
         return false
       }
     }
+    fatalError()
   }
 }
 

stdlib/public/core/StringComparison.swift

@@ -265,6 +265,7 @@ private func _findBoundary(
 
     idx &-= _utf8ScalarLength(utf8, endingAt: idx)
   }
+  fatalError()
 }
 
 @frozen

stdlib/public/core/StringUTF16View.swift

@@ -945,6 +945,7 @@ extension String.UTF16View {
 
         readIdx &+= len
       }
+      fatalError()
     }
   }
 

stdlib/public/core/UTF16.swift

@@ -256,6 +256,7 @@ extension Unicode.UTF16 {
         return (utf16Count, utf16BitUnion < 0x80)
       }
     }
+    fatalError()
   }
 }
 

stdlib/public/core/Unicode.swift

@@ -585,6 +585,7 @@ public func transcode<
     }
     OutputEncoding.encodedReplacementCharacter.forEach(processCodeUnit)
   }
+  fatalError()
 }
 
 /// Instances of conforming types are used in internal `String`