[clang-format] Add BitFieldColonSpacing option

This new option allows controlling if there should be spaces around
the ':' in a bitfield declaration.

BitFieldColonSpacing accepts four different values:

  // "Both" - default
  unsigned bitfield : 5
  unsigned bf2      : 5  // AlignConsecutiveBitFields=true

  // "None"
  unsigned bitfield:5
  unsigned bf2     :5

  // "Before"
  unsigned bitfield :5
  unsigned bf2      :5

  // "After"
  unsigned bitfield: 5
  unsigned bf2     : 5

Differential Revision: https://reviews.llvm.org/D84090