Debug Info: Provide a means to update the members of a composite type

This will be used to factor out some uses of magic number operand offsets
inside Clang where these fields were updated in an effort to resolve forward
declarations/circular references.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178078 91177308-0d34-0410-b5e6-96231b3b80d8