clang-format: Add a new brace style "custom" as well as flags to

control the individual braces. The existing choices for brace wrapping
are now merely presets for the different flags that get expanded upon
calling the reformat function.

All presets have been chose to keep the existing formatting, so there
shouldn't be any difference in formatting behavior.

Also change the dump_format_style.py to properly document the nested
structs that are used to keep these flags discoverable among all the
configuration flags.

llvm-svn: 248802