Apply modernize-use-default to clang.

Summary: Replace empty bodies of default constructors and destructors with '= default'.

Reviewers: bkramer, klimek

Subscribers: klimek, alexfh, cfe-commits

Differential Revision: http://reviews.llvm.org/D13890

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@250822 91177308-0d34-0410-b5e6-96231b3b80d8