refactor: Unify+simplify DWARFCompileUnit ctor+Clear() into in-class initializers + Extract()

It has no functionality effect.

I was concerned about the worse performance of DWARFDebugInfo::Parse this way
of allocating+destroying a CU for each iteration but I see it is now used only
by DWARFDebugInfo::Dump so that is no longer a problem.

Differential revision: https://reviews.llvm.org/D40212

llvm-svn: 319359