Revert "Use value semantics to manage DbgVariables rather than dynamic allocation/pointers."

This reverts commit r206780.

This commit was regressing gdb.opt/inline-locals.exp in the GDB 7.5 test
suite. Reverting until I can fix the issue.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206867 91177308-0d34-0410-b5e6-96231b3b80d8