[cxx-interop][overlay] Revert memcpy string init approach.

For some reason this isn't working, so I reverted to the previous approach. We can try to re-visit this in the future for a potential perf improvement.

Author: zoecarver

stdlib/public/Cxx/std/String.swift

@@ -12,12 +12,9 @@
 
 extension std.string {
   public init(_ string: String) {
-    let count = string.count
-    self.init(count, value_type(0))
-    var string = string
-    string.withUTF8 {
-      memcpy(UnsafeMutableRawPointer(mutating: UnsafeRawPointer(__dataUnsafe()!)),
-             UnsafeMutableRawPointer(mutating: $0.baseAddress), count)
+    self.init()
+    for char in string.utf8 {
+      self.push_back(value_type(char))
     }
   }
 }