[lmi] [io]fstream move ctors

[Greg Chicares]

Vadim--Please help me understand why clang considers the [io]fstream
move ctors to be deleted (cf. the [squashed] commit below). C++20
(N4861) [ifstream] says:
  // 29.9.3.1, constructors
  basic_ifstream(basic_ifstream&& rhs);
I assume that it wouldn't be mentioned there if it were deleted;
otherwise, clang would issue a warning for the specification in the
standard itself.

Isn't that the "corresponding base class" of our own fs::ifstream?

commit b59e479bf1c00da63e4ce49b939e97e86c2417c6
Author: Vadim Zeitlin <vadim@tt-solutions.com>
Date:   2021-02-26T16:37:56+00:00

    Delete fs::ifstream and ofstream move ctors
    
    These ctors don't exist because they're deleted in the corresponding
    base classes, and explicitly defaulting them is misleading (because it
    might lead one to believe that they are available, when this is not the
    case) and results in -Wdefaulted-function-deleted from clang.
[...] 
     ifstream(ifstream const&) = delete;
-    ifstream(ifstream&&) = default;
+    ifstream(ifstream&&) = delete;