Add unique_fd::operator{==,!=} overloads that take a unique_fd.

With C++20, any use of the existing overloads with two unique_fd
arguments is likely to become ambiguous:
http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20200113/301880.html

Newer versions of clang complain about this ambiguity. Fix the error by
adding overloads that take two unique_fds.

Bug: 145916209
Change-Id: I18a292827d8841b6d24f948682123ab54dc7aaca
diff --git a/base/include/android-base/unique_fd.h b/base/include/android-base/unique_fd.h
index 1605daf..c4a0aad 100644
--- a/base/include/android-base/unique_fd.h
+++ b/base/include/android-base/unique_fd.h
@@ -116,6 +116,8 @@
   bool operator<(int rhs) const { return get() < rhs; }
   bool operator==(int rhs) const { return get() == rhs; }
   bool operator!=(int rhs) const { return get() != rhs; }
+  bool operator==(const unique_fd_impl& rhs) const { return get() == rhs.get(); }
+  bool operator!=(const unique_fd_impl& rhs) const { return get() != rhs.get(); }
 
   // Catch bogus error checks (i.e.: "!fd" instead of "fd != -1").
   bool operator!() const = delete;