Tidyup: Move HasLoc instances into their own module from compiler/GHC/Iface/Ext/Ast.hs

This is a follow-up from !10743 (closed) which moved the HasLoc class from compiler/GHC/Iface/Ext/Ast.hs to compiler/GHC/Parser/Annotation.hs.

This had the unfortunate effect of introducing orphans in Ast.hs. Remove the orphan instances, probably by moving the HasLoc instance into its own module, as discussed in !10743 (comment 508331)