Don't pre-populate the set of keys in the map with variable locations history.

Current implementation of calculateDbgValueHistory already creates the
keys in the expected order (user variables are listed in order of appearance),
and should do so later by contract.

No functionality change.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209690 91177308-0d34-0410-b5e6-96231b3b80d8