Update C++ styleguide

Major changes:
- discourage const references when the input parameter must outlive the
  call
- drop ban on mutable references
- allow rvalue-qualified methods in certain situations
- allow C-style cast when casting to void
- disallow postfix increment/decrement operators unless postfix
  semantics are required
- allow designated initializers in C++20-compliant form
- disallow macro-style naming for enumerator constant for new code
- disallow spaces inside parentheses for conditionals

The change itself is largely automated, with a few manual adjustments:
- manually restore the TOC placeholder
- removed trailing whitespace