This reverts commit febf9ce949c0bc87a8bf7ab3ddae4deb28636c9d.

This reverts commit d72d54ebb0b7784f5616edc02910dbd9cce0b2a4.

This reverts commit 59bc2e183e7aef5e45c06aed6965de0ac52d7b06.

This reverts commit 19c5a3613fee71878ba2af0339eed325a5916089.

Fully emulating type_caster_base-related behavior: trying shared_from_this also for unowned pointees.

enable_shared_from_this_from_raw_ptr_take_ownership_guard: better static_assert that also triggers for derived classes.

Restoring init_holder overload for std::enable_shared_from_this and original tests/test_smart_ptr.py.

Breaking up test_shared_ptr_from_this_and_references into smaller subtests. Skipping the only test that generates an ASAN heap-use-after-free.

Experiments: 1. disabling enable_shared_from_this, 2. using smart_holder in test_class_sh_shared_from_this.cpp

* Allow casting from None to a custom object, closes #2778

* ci.yml patch from the smart_holder branch for full CI coverage.

This also disables catch for some other builds that happen to be in the same
matrix. But there are still plenty of builds running with DOWNLOAD_CATCH=ON.

Workaround for a windows-only download issue that started yesterday (2020-06-23):

-- Downloading catch v2.13.2...
CMake Error at tools/FindCatch.cmake:40 (message):
  Could not download
  https://github.com/philsquared/Catch/releases/download/v2.13.2/catch.hpp
Call Stack (most recent call first):
  tools/FindCatch.cmake:59 (_download_catch)
  tests/pure_cpp/CMakeLists.txt:1 (find_package)

* Updating and slightly enhancing instructions for running clang-tidy.

* Making documented commands identical to workflow commands. Adding comment in workflow file pointing to documentation.