Move DwarfUnit::constructVariableDIE down to DwarfCompileUnit, since it's only needed there.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219418 91177308-0d34-0410-b5e6-96231b3b80d8
Sink DwarfDebug.cpp::constructVariableDIE into DwarfCompileUnit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219419 91177308-0d34-0410-b5e6-96231b3b80d8