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

llvm-svn: 217748