Reapply "IR: Simplify DIBuilder's HeaderBuilder API, NFC"

This reverts commit r226542, effectively reapplying r226540.  This time,
initialize `IsEmpty` in the copy and move constructors as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226545 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed