[clang-format] Do not format likely xml

Summary:
This patch detects the leading '<' in likely xml files and stops formatting in
that case. A recent use of a Qt xml file with a .ts extension triggered this:
http://doc.qt.io/qt-4.8/linguist-ts-file-format.html

Reviewers: djasper

Reviewed By: djasper

Subscribers: sammccall, cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D37136

llvm-svn: 311999