[String] DCE and drop inlinable

Remove some more inlinable annotations and drop dead code.

stdlib/public/core/StringUTF16View.swift

@@ -356,7 +356,6 @@ extension String.UTF16View.Index {
   ///   position in `unicodeScalars`, this method returns `nil`. For example,
   ///   an attempt to convert the position of a UTF-16 trailing surrogate
   ///   returns `nil`.
-  @inlinable // FIXME(sil-serialize-all)
   public func samePosition(
     in unicodeScalars: String.UnicodeScalarView
   ) -> String.UnicodeScalarIndex? {