[clang-format] Ignore case and stable sort using-declarations

Summary:
This ignores case while sorting using-declarations, fixing a case where `_` would appear between lowercase and uppercase characters.
It also applies stable sort, so that replacements for the exact same using declarations are not generated.

Reviewers: klimek, alexfh

Reviewed By: alexfh

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D37263

llvm-svn: 313963