DebugInfo: Emit DW_TAG_subprogram's DW_AT_high_pc as an offset from the low_pc

This removes a relocation from each subprogram, reducing link times,
etc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203187 91177308-0d34-0410-b5e6-96231b3b80d8