Move the DataLayout to the generic TargetMachine, making it mandatory.

Summary:
I don't know why every singled backend had to redeclare its own DataLayout.
There was a virtual getDataLayout() on the common base TargetMachine, the
default implementation returned nullptr. It was not clear from this that
we could assume at call site that a DataLayout will be available with
each Target.

Now getDataLayout() is no longer virtual and return a pointer to the
DataLayout member of the common base TargetMachine. I plan to turn it into
a reference in a future patch.

The only backend that didn't have a DataLayout previsouly was the CPPBackend.
It now initializes the default DataLayout. This commit is NFC for all the
other backends.

Test Plan: clang+llvm ninja check-all

Reviewers: echristo

Subscribers: jfb, jholewinski, llvm-commits

Differential Revision: http://reviews.llvm.org/D8243

From: Mehdi Amini <mehdi.amini@apple.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231987 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/Mips/MipsTargetMachine.h

@@ -31,7 +31,6 @@ class MipsTargetMachine : public LLVMTargetMachine {
   std::unique_ptr<TargetLoweringObjectFile> TLOF;
   // Selected ABI
   MipsABIInfo ABI;
-  const DataLayout DL; // Calculates type size & alignment
   MipsSubtarget *Subtarget;
   MipsSubtarget DefaultSubtarget;
   MipsSubtarget NoMips16Subtarget;
@@ -47,7 +46,6 @@ public:
 
   TargetIRAnalysis getTargetIRAnalysis() override;
 
-  const DataLayout *getDataLayout() const override { return &DL; }
   const MipsSubtarget *getSubtargetImpl() const override {
     if (Subtarget)
       return Subtarget;