Use a single data structure to store all user variables in DwarfDebug

Summary:
Get rid of UserVariables set, and turn DbgValues into MapVector
to get a fixed ordering, as suggested in review for http://reviews.llvm.org/D3573.

Test Plan: llvm regression tests

Reviewers: dblaikie

Reviewed By: dblaikie

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D3579

llvm-svn: 207720