[Sema] Don't work around a malformed AST

We created a malformed TemplateSpecializationType: it was dependent but
had a RecordType as it's canonical type.  This would lead getAs to
crash.  r249090 worked around this but we should fix this for real by
providing a more appropriate template specialization type as the
canonical type.

This fixes PR24246.

llvm-svn: 253495