CMake 3.27 deprecates compatibility with CMake < 3.5.  Update tests that
do not cover older interfaces to avoid the deprecation warning.

Follow the pattern from:

* commit 7b07ccdd2b (Tests/*Only: Update cmake_minimum_required versions,
                     2020-06-15, v3.19.0-rc1~629^2~1)

* commit 72e7c45e98 (Tests: Bump CMake minimum required in tests to 2.8.12,
                     2020-12-22, v3.20.0-rc1~224^2)

* commit f6b4db365a (Tests: bump cmake_minimum_required version to 2.8.12,
                     2021-04-04, v3.21.0-rc1~372^2)

Also remove explicit `cmake_policy` settings made redundant by the
version.

Fixes: #17904

This is typically how projects include them, and cl's `-external:{I,W}`
flags suppress warnings only when included through angle brackets.

Since 3.19, CMake generates a deprecation warning when using a minimum
version less than 2.8.12.  This eliminates those warnings generated
during tests, which are typically hidden from the user and developer but
are being generated nonetheless.

Run the `clang-format.bash` script to update our C and C++ code to a new
include order `.clang-format`.  Use `clang-format` version 6.0.

An effect of the `-isystem` flag is to search the directory after those
specified via `-I` flags.  Make behavior more consistent on compilers
that do not have any `-isystem` flag by explicitly moving system include
directories to the end.

When evaluating include directories during export to a `try_compile`
test project, thread the compile language through to the generator
expression evaluator so it can support `$<COMPILE_LANGUAGE:...>`.

Issue: #17811