compiler/semtypes: improve lineinfo for exported object fields (#12495)

The line info should now points to the `a`, not the `*`, like this:
    a*: string
    ^
Additionally this fixes nimsuggest's highlighting of exported object
fields.

nimsuggest/tests/tobj_highlight.nim

@@ -0,0 +1,11 @@
+type
+  O = object
+    a*: int#[!]#
+
+discard """
+$nimsuggest --tester $file
+>highlight $1
+highlight;;skType;;2;;2;;1
+highlight;;skType;;3;;8;;3
+highlight;;skField;;3;;4;;1
+"""