DebugInfo: Adapt to loss of DITypeRef in LLVM r267296

LLVM stopped using MDString-based type references, and DIBuilder no
longer fills 'retainedTypes:' with every DICompositeType that has an
'identifier:' field.   There are just minor changes to keep the same
behaviour in CFE.

Leaving 'retainedTypes:' unfilled has a dramatic impact on the output
order of the IR though.  There are a huge number of testcase changes,
which were unfortunately not really scriptable.

llvm-svn: 267297