DebugInfo: Restrict DW_AT_high_pc encoding as data4 offset to DWARF 4 as per spec

Code review feedback to r203187 from Oliver Stannard. Thanks!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203256 91177308-0d34-0410-b5e6-96231b3b80d8