DebugInfo: Remove CompileUnit::constructTypeDIEImpl now that it's just a simple wrapper again.

r195698 moved the type unit checking up into getOrCreateTypeDIE so
remove the redundant check and fold the functions back together again.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195700 91177308-0d34-0410-b5e6-96231b3b80d8