Add "-Wc++11-inline-namespace" so that libc++ can use -pedantic in C++03.

Summary: The title says it all.

Reviewers: rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D13790

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@250477 91177308-0d34-0410-b5e6-96231b3b80d8