Add DIBuilder functions to build RAUWable DIVariables and DIFunctions.

Summary: These will be used to implement support for useful forward declarartions.

Reviewers: echristo, dblaikie, aprantl

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5328

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217949 91177308-0d34-0410-b5e6-96231b3b80d8