Rename TD_Confirm to TD_TypeInfo.

The old name probably was a reference to the fact that the
metadata node is used to _confirm_ that a metadata record
found by name really belongs to a given TypeInfo instance,
but I found it to be rather non-intuitive.
Removed tautological checks.

They were probably used during initial development of the
pass to be able to disable emission of the global reference
by setting TD_TypeInfo to -1.
Remove USE_METADATA.

It has been on by default for quite some time now.
Avoid emitting invalid IR for function type TypeInfo metadata.

This fixes LLVM debug builds of the DMD testsuite.