STLExtras: Add convenience is_contained() function.

This commit adds a convenience is_contained() function
which checks if an element exists in a container. It is part of a larger
series of patches adding an MPI checker to the clang static analyzer.

Reviewers: dblaikie,bkramer

A patch by Alexander Droste!

Differential Revision:http://reviews.llvm.org/D16053

llvm-svn: 271757