Introduce the DWARFUnitSection abstraction.

A DWARFUnitSection is the collection of Units that have been extracted from
the same debug section.

By embeding a reference to their DWARFUnitSection in each unit, the DIEs
will be able to resolve inter-unit references by interrogating their Unit's
DWARFUnitSection.

This is a minimal patch where the DWARFUnitSection is-a SmallVector of Units,
thus exposing exactly the same interface as before. Followup-up patches might
change from inheritance to composition in order to expose only the wanted
DWARFUnitSection abstraction.

    Differential Revision: http://reviews.llvm.org/D5310
Move replaceAllUsesWith() from DIType to DIDescriptor.

RAUW was only used on DIType to merge declarations and full definitions
of types. In order to support the same functionality for functions and
global variables, move the function up type DI type hierarchy to the
common parent of DIType, DISubprogram and DIVariable which is
DIDescriptor.

This functionality will be exercized when we add the code to emit
imported declarations for forward declared function/variables.

Reviewers: echristo, dblaikie, aprantl

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5325
Fix DebugInfo replaceAllUsesWith.

Summary:
replaceAllUsesWith had been modified to allow a DbgNode value to be
replaced by itself. In that case a new node is created by copying the
current DbgNode and the copy is used as replacement value.

When that copying happens, the value stored in this->DbgNode at the end
of RAUW would be a reference to the Node that has just been deleted.

This doesn't produce any bug right now, because the DI node on which we
call RAUW won't be used again.

Reviewers: dblaikie, echristo, aprantl

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5326