[clang-format] Ensure dump_format_style.py can generate...

[clang-format] Ensure dump_format_style.py can generate ClangFormatStyleOptions.rst without manual intervention

Summary:
This revision is the last in a series of revisions to return `clang/doc/tools/dump_format_style.py` to be being able to parse Format.h without needing to manually merge the ClangFormatStyleOptions.rst file.

The final modification to dump_format_style.py is needed following the addition of a  nested enumeration inside a nested structure following the introduction of {D68296}

Prior  related revisions will allow for a fully clang-formatted `clang/include/clang/Format/Format.h` to once again be used at the source.
{D69951}
{D69433}
{D69404}

Reviewers: mitchell-stellar, klimek, sammccall, owenpan

Reviewed By: mitchell-stellar

Subscribers: cfe-commits

Tags: #clang-format, #clang

Differential Revision: https://reviews.llvm.org/D70003