Re-instate "stdlib: Add some inlining attributes to help the optimizer getting the right inlining decisions."

This re-instates commit 8b0e36779e

stdlib/public/core/StringUnicodeScalarView.swift

@@ -41,6 +41,7 @@ extension String {
         self.idx = pos
         self.core = core
       }
+      @inline(__always)
       mutating func next() -> UTF16.CodeUnit? {
         if idx == core.endIndex {
           return nil
@@ -61,6 +62,7 @@ extension String {
       ///
       /// - Precondition: The next value is representable.
       @warn_unused_result
+      @inline(__always)
       public func successor() -> Index {
         var scratch = _ScratchIterator(_core, _position)
         var decoder = UTF16()