DwarfCompileUnit: Initialize DebugInfoOffset.

While not strictly necessary (the class has an invariant that
"setDebugInfoOffset" is called before "getDebugInfoOffset" - anyone
client that actually gets the default zero offset is buggy/broken) this
is consistent with the code as originally written and the removal of the
initialization was an accident in r195166.

Suggested by Manman Ren.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195263 91177308-0d34-0410-b5e6-96231b3b80d8