Add -fbracket-depth=N, analogous to -ftemplate-depth= and -fconstexpr-depth=,
to control the check for the C 5.2.4.1 / C++ [implimits] restriction on nesting
levels for parentheses, brackets and braces.

Some code with heavy macro use exceeds the default limit of 256, but we don't
want to increase it generally to avoid stack overflow on stack-constrained
systems.