diagnose "unexpected EOF" at EOF

this needs a string comparison because there seems to be no other way to
get that information out of bison. usually the location info is going to
be correct (pointing at a bad token), but since EOF isn't a token as
such it'll be wrong in that this case.

this hasn't shown up much so far because a single line ending *is* a
token, so any file formatted in the usual manner (ie, ending in a line
ending) would have its EOF position reported correctly.

tests/functional/lang/parse-fail-eof-pos.nix

@@ -0,0 +1,2 @@
+(
+# no content