Fix #19377 by using lookupLOcc when desugaring TH-quoted ANNs

Previously, the desugarer was looking up names referenced in TH-quoted `ANN`s
by using `globalVar`, which would allocate a fresh TH `Name`. In effect, this
would prevent quoted `ANN`s from ever referencing the correct identifier
`Name`, leading to #19377. The fix is simple: instead of `globalVar`, use
`lookupLOcc`, which properly looks up the name of the in-scope identifier.

Fixes #19377.