Add enum value negative check

There was one already at parse time, this adds a check later so that
cases like overflows or internal enums with negative values get caught.

testing/btest/Baseline/language.enum-negative/output

@@ -0,0 +1,3 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+error in <...>/enum-negative.zeek, line 5: enumerator is not a count constant
+error in <...>/enum-negative.zeek, line 6: enumerator value cannot be negative

testing/btest/language/enum-negative.zeek

@@ -0,0 +1,7 @@
+# @TEST-EXEC-FAIL: zeek -b %INPUT >output 2>&1
+# @TEST-EXEC: TEST_DIFF_CANONIFIER="$SCRIPTS/diff-remove-abspath" btest-diff output
+
+type my_enum: enum {
+	explicitly_negative = -1,
+	overflow = 9223372036854775808,
+};