Prune include of DataLayout.h from include/clang/Basic/TargetInfo.h. NFC

Summary:
Use a forward declaration of DataLayout instead of including
DataLayout.h in clangs TargetInfo.h. This reduces include
dependencies toward DataLayout.h (and other headers such as
DerivedTypes.h, Type.h that is included by DataLayout.h).

Needed to move implemantation of TargetInfo::resetDataLayout
from TargetInfo.h to TargetInfo.cpp.

Reviewers: rnk

Reviewed By: rnk

Subscribers: jvesely, nhaehnle, cfe-commits, llvm-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69262

llvm-svn: 375438