DebugInfo: more support for mutating DICompositeType to reduce magic number usage in Clang

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178090 91177308-0d34-0410-b5e6-96231b3b80d8
Make DIBuilder::createClassType more type safe by returning DICompositeType rather than DIType

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178091 91177308-0d34-0410-b5e6-96231b3b80d8