Sink DwarfDebug::attachLowHighPC into DwarfCompileUnit

One of many things to sink down into DwarfCompileUnit to allow handling
of subprograms in both the skeleton and dwo CU under Fission.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219058 91177308-0d34-0410-b5e6-96231b3b80d8
Reformatting accidentally left out of r219057

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219059 91177308-0d34-0410-b5e6-96231b3b80d8
Sink DwarfDebug::updateSubprogramScopeDIE into DwarfCompileUnit

This requires exposing some of the current function state from
DwarfDebug. I hope there's not too much of that to expose as I go
through all the functions, but it still seems nicer to expose singular
data down to multiple consumers, than have consumers expose raw mapping
data structures up to DwarfDebug for building subprograms.

Part of a series of refactoring to allow subprograms in both the
skeleton and dwo CUs under Fission.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219060 91177308-0d34-0410-b5e6-96231b3b80d8