Remove DIBuilder cache of variable TheCU and change the few
uses that wanted it. Also change the interface for createCompileUnit
to compensate. Fix comments that refer to TheCU as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186637 91177308-0d34-0410-b5e6-96231b3b80d8