CGDebugInfo: Factor out a getOrCreateStandaloneType() method.

Usually debug info is created on the fly while during codegen.
With this API it becomes possible to create standalone debug info
for types that are not referenced by any code, such as emitting debug info
for a clang module or for implementing something like -gfull.
Because on-the-fly debug info generation may still insert retained types
on top of them, all RetainedTypes are uniqued in CGDebugInfo::finalize().

llvm-svn: 246210