Add an initializer for Span without lifetime dependence

Without this, supported old compilers which don't recognize LifetimeDependence feature but
recognize the NonEscapableTypes feature will run into lifetime dependence inference errors
on the implicit initializer generated for the stdlib's swiftinterface

Author: meg-gupta

stdlib/public/core/Span/RawSpan.swift

@@ -44,6 +44,15 @@ public struct RawSpan: ~Escapable, Copyable, BitwiseCopyable {
   @usableFromInline
   internal let _count: Int
 
+  /// FIXME: Remove once supported old compilers can recognize lifetime dependence 
+  @_unsafeNonescapableResult
+  @_alwaysEmitIntoClient
+  @inline(__always)
+  internal init() {
+    _pointer = nil
+    _count = 0
+  }
+
   /// Unsafely create a `RawSpan` over initialized memory.
   ///
   /// `pointer` must point to a region of `byteCount` initialized bytes,

stdlib/public/core/Span/Span.swift

@@ -46,6 +46,15 @@ public struct Span<Element: ~Copyable & ~Escapable>
   @usableFromInline
   internal let _count: Int
 
+  /// FIXME: Remove once supported old compilers can recognize lifetime dependence 
+  @_unsafeNonescapableResult
+  @_alwaysEmitIntoClient
+  @inline(__always)
+  internal init() {
+    _pointer = nil
+    _count = 0
+  }
+
   /// Unsafely create a `Span` over initialized memory.
   ///
   /// `pointer` must point to a region of `count` initialized instances,