Debug Info: move class definition of DIRef.

Debug Info: move class definition of DIRef.

Definition of DIRef used to require the full definition of DIType because
of usage of DIType::isType in DIRef::resolve. We now use DIDescriptor::isType
instead to remove the requirement and move definition of DIRef before DIType.

With this, we can move the definition of DIType::getContext to the header
file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190540 91177308-0d34-0410-b5e6-96231b3b80d8