[clang-tidy] Enhance clang-tidy readability-simplify-boolean-expr check...

Enhance clang-tidy readability-simplify-boolean-expr check to handle chained
conditional assignment and chained conditional return.

Based on feedback from applying this tool to the clang/LLVM codebase, this
changeset improves the readability-simplify-boolean-expr check so that
conditional assignment or return statements at the end of a chain of if/else if
statements are left unchanged unless a configuration option is supplied.

http://reviews.llvm.org/D8996

Patch by Richard Thomson!

llvm-svn: 237541