Remove access to the DataLayout in the TargetMachine

Summary:
Replace getDataLayout() with a createDataLayout() method to make
explicit that it is intended to create a DataLayout only and not
accessing it for other purpose.

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

Reviewers: echristo

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

Differential Revision: http://reviews.llvm.org/D11103

(cherry picked from commit 5609fc56bca971e5a7efeaa6ca4676638eaec5ea)

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 243114