cmd/compile: fix "previous" position info for duplicate switch cases
Because the Node AST represents references to declared objects (e.g.,
variables, packages, types, constants) by directly pointing to the
referred object, we don't have use-position info for these objects.
For switch statements with duplicate cases, we report back where the
first duplicate value appeared. However, due to the AST
representation, if the value was a declared constant, we mistakenly
reported the constant declaration position as the previous case
position.
This CL reports back against the 'case' keyword's position instead, if
there's no more precise information available to us.
It also refactors code to emit the same "previous at" error message
for duplicate values in map literals.