[clang-tidy] add missing assignment operations in hicpp-signed-bitwise

This patch resolves the bug https://bugs.llvm.org/show_bug.cgi?id=36963.

- implement missing assignment operators for hicpp-signed-bitwise
- mention fix in release notes

Reviewers:
aaron.ballman, hokein, alexfh

Differential: https://reviews.llvm.org/D45414
llvm-svn: 329789