[clang-format] Sort using-declarations case sensitively with a special case for '_'

Summary:
This makes clang-format sort using declarations case-sensitive with the
exception that '_' comes just before 'A'. This is better than the current case
insensitive version, because it groups uppercase names in the same namespace
together.

Reviewers: bkramer

Reviewed By: bkramer

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D39549

llvm-svn: 317325