Add _overrideLifetime to Span APIs

The implementation previously took advantage of a missing diagnostic. They
returned a borrowed Span with a dependency on a copied argument. This illegally
promotes the dependency. This is what the author intended to do (the
implementation was correct) but the compiler can't know that so the code needs
to be explicit about overriding the lifetime.

Author: atrick

stdlib/public/core/Span/RawSpan.swift

@@ -711,7 +711,8 @@ extension RawSpan {
   public func _extracting(first maxLength: Int) -> Self {
     _precondition(maxLength >= 0, "Can't have a prefix of negative length")
     let newCount = min(maxLength, byteCount)
-    return unsafe Self(_unchecked: _pointer, byteCount: newCount)
+    let newSpan = unsafe Self(_unchecked: _pointer, byteCount: newCount)
+    return unsafe _overrideLifetime(newSpan, copying: self)
   }
 
   /// Returns a span over all but the given number of trailing bytes.
@@ -734,7 +735,8 @@ extension RawSpan {
     _precondition(k >= 0, "Can't drop a negative number of bytes")
     let droppedCount = min(k, byteCount)
     let count = byteCount &- droppedCount
-    return unsafe Self(_unchecked: _pointer, byteCount: count)
+    let newSpan = unsafe Self(_unchecked: _pointer, byteCount: count)
+    return unsafe _overrideLifetime(newSpan, copying: self)
   }
 
   /// Returns a span containing the trailing bytes of the span,

stdlib/public/core/Span/Span.swift

@@ -764,7 +764,8 @@ extension Span where Element: ~Copyable {
   public func _extracting(first maxLength: Int) -> Self {
     _precondition(maxLength >= 0, "Can't have a prefix of negative length")
     let newCount = min(maxLength, count)
-    return unsafe Self(_unchecked: _pointer, count: newCount)
+    let newSpan = unsafe Self(_unchecked: _pointer, count: newCount)
+    return unsafe _overrideLifetime(newSpan, copying: self)
   }
 
   /// Returns a span over all but the given number of trailing elements.
@@ -786,7 +787,8 @@ extension Span where Element: ~Copyable {
   public func _extracting(droppingLast k: Int) -> Self {
     _precondition(k >= 0, "Can't drop a negative number of elements")
     let droppedCount = min(k, count)
-    return unsafe Self(_unchecked: _pointer, count: count &- droppedCount)
+    let newSpan = unsafe Self(_unchecked: _pointer, count: count &- droppedCount)
+    return unsafe _overrideLifetime(newSpan, copying: self)
   }
 
   /// Returns a span containing the final elements of the span,

stdlib/public/core/UTF8Span.swift

@@ -181,7 +181,8 @@ extension UTF8Span {
   public var span: Span<UInt8> {
     @lifetime(copy self)
     get {
-      unsafe Span(_unchecked: _unsafeBaseAddress, count: self.count)
+      let newSpan = unsafe Span<UInt8>(_unchecked: _unsafeBaseAddress, count: self.count)
+      return unsafe _overrideLifetime(newSpan, copying: self)
     }
   }