Some tests fail because Xcode runs `POST_BUILD` commands before signing
the binaries they run.  Tell the linker to perform ad-hoc codesign even
though Xcode normally tells it not to.

Other tests fail because `install_name_tool` does not revise ad-hoc
signatures without the codesign `linker-signed` flag.  Add that flag
ourselves where needed by our tests.

For now these changes help our test suite pass so we can use it to cover
everything else.  Both of these cases may need further investigation to
update CMake to help projects in general.

Issue: #21845, #21854

These were added by commit e29a92f525 (Xcode: Make BundleTests
compatible with Xcode 11, 2019-09-01, v3.16.0-rc1~158^2).  The
real problem is that the `.app` is constructed with text files
under the `.app/Contents/MacOS/` folder.  Move those, and drop
the special no-signing configuration.

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.

For `MACOSX_BUNDLE` targets we generate an `Info.plist` automatically
and add it to the sources presented to Xcode.  Avoid mutating the
original target's list of sources to achieve this.  Otherwise when we
generate the same target again (e.g. in a sub-project's Xcode file) it
will look different than the first time and possibly break invariants.

Fixes: #19114

Run the `clang-format.bash` script to update all our C and C++ code to a
new style defined by `.clang-format`.  Use `clang-format` version 6.0.

* If you reached this commit for a line in `git blame`, re-run the blame
  operation starting at the parent of this commit to see older history
  for the content.

* See the parent commit for instructions to rebase a change across this
  style transition commit.

Run the `Utilities/Scripts/clang-format.bash` script to update
all our C++ code to a new style defined by `.clang-format`.
Use `clang-format` version 3.8.

* If you reached this commit for a line in `git blame`, re-run the blame
  operation starting at the parent of this commit to see older history
  for the content.

* See the parent commit for instructions to rebase a change across this
  style transition commit.

Sort include directives within each block (separated by a blank line) in
lexicographic order (except to prioritize `sys/types.h` first).  First
run `clang-format` with the config file:

    ---
    SortIncludes: false
    ...

Commit the result temporarily.  Then run `clang-format` again with:

    ---
    SortIncludes: true
    IncludeCategories:
      - Regex:    'sys/types.h'
        Priority: -1
    ...

Commit the result temporarily.  Start a new branch and cherry-pick the
second commit.  Manually resolve conflicts to preserve indentation of
re-ordered includes.  This cleans up the include ordering without
changing any other style.

Use the following command to run `clang-format`:

    $ git ls-files -z -- \
        '*.c' '*.cc' '*.cpp' '*.cxx' '*.h' '*.hh' '*.hpp' '*.hxx' |
      egrep -z -v '(Lexer|Parser|ParserHelper)\.' |
      egrep -z -v '^Source/cm_sha2' |
      egrep -z -v '^Source/(kwsys|CursesDialog/form)/' |
      egrep -z -v '^Utilities/(KW|cm).*/' |
      egrep -z -v '^Tests/Module/GenerateExportHeader' |
      egrep -z -v '^Tests/RunCMake/CommandLine/cmake_depends/test_UTF-16LE.h' |
      xargs -0 clang-format -i

This selects source files that do not come from a third-party.

Inspired-by: Daniel Pfeifer <daniel@pfeifer-mail.de>

The test just wants a source file from outside its tree.
Use README.rst instead.