Make ExecutionEngine owning a DataLayout

Summary:
This change is part of a series of commits dedicated to have a single
DataLayout during compilation by using always the one owned by the
module.

The ExecutionEngine will act as an exception and will be unsafe to
be reused across context. We don't enforce this rule but undefined
behavior can occurs if the user tries to do it.

Reviewers: lhames

Subscribers: echristo, llvm-commits, rafael, yaron.keren

Differential Revision: http://reviews.llvm.org/D11110

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 242387