[cxx-interop] Fix a potential dangling pointer in String(cxxString:)

`cxxString` might get deallocated immediately after `c_str()` call, which would mean that the pointer passed to `String(cString:)` points to invalid memory.

We haven't actually seen this, but let's preemptively add an explicit `withExtendedLifetime` call to avoid running into this in the future.

Author: egorzhdan

stdlib/public/Cxx/String.swift

@@ -22,5 +22,6 @@ extension std.string {
 extension String {
   public init(cxxString: std.string) {
     self.init(cString: cxxString.c_str())
+    withExtendedLifetime(cxxString) {}
   }
 }