[clang-format] Don’t propagate AvoidBinPacking into argument

subexpressions

This is an attempt to fix the issue described in a recent email:
http://lists.llvm.org/pipermail/cfe-dev/2017-April/053632.html

Patch by jtbandes. Thank you!
Review: https://reviews.llvm.org/D32475

llvm-svn: 302427