Fix clang-format's detection of structured bindings.

Correctly determine when [ is part of a structured binding instead of a
lambda.

To be able to reuse the implementation already available, this patch also:
- sets the Previous link of FormatTokens in the UnwrappedLineParser
- moves the isCppStructuredBinding function into FormatToken

Before:
  auto const const &&[x, y] { A *i };

After:
  auto const const && [x, y]{A * i};

Fixing formatting of the type of the structured binding is still missing.

llvm-svn: 313742