[clang-format] Fixed edge-case with SpacesInSquareBrackets with trailing bare "&" lambda capture.

Summary:
Lambda captures allow for a lone `&` capture, so `&]` needs to be properly handled.

`int foo = [& ]() {}` is fixed to give `int foo = [ & ]() {}`

Reviewers: MyDeveloperDay

Reviewed by: MyDeveloperDay

Subscribers: cfe-commits

Tags: #clang, #clang-format

Differential Revision: https://reviews.llvm.org/D70249