[ScopDetectDiagnostics] Do not format unnamed array names

Formatting unnamed array names is expensive in LLVM as the this requires
deriving the numbered virtual instruction name (e.g., %12) for an llvm::Value,
which is currently not implemented efficiently. As instruction numberes anyhow
do not really carry a lot of information for the user, we just print 'unknown'
instead.

This change reduces the scop detection time from 24 to 19 seconds, for one of
our large-scale inputs. This is a reduction by 21%.

llvm-svn: 294894