Accept UnaryMinusExpression as class parameter type

Previously, class parameters of the form `Integer[-1] $param` would fail
compilation, because the value `-1` was lexed as a UnaryMinusExpression
containing a LiteralInteger. And since the LiteralEvaluator didn't implement the
`literal_UnaryMinusExpression` method, the visitor called `literal_XXX` for each
ancestor class, until reaching `literal_Object`, which always raises.

This adds the `literal_UnaryMinusExpression` method and returns -1 times
the expression it wraps. This is similar to how the TypeParser interprets
UnaryMinusExpressions[1].

[1] https://github.com/puppetlabs/puppet/blob/8.5.0/lib/puppet/pops/types/type_parser.rb#L161

lib/puppet/pops/evaluator/literal_evaluator.rb

@@ -76,6 +76,10 @@ def literal_AccessExpression(o)
     o.keys.map { |v| literal(v) }
   end
 
+  def literal_UnaryMinusExpression(o)
+    -literal(o.expr)
+  end
+
   def literal_ConcatenatedString(o)
     # use double quoted string value if there is no interpolation
     throw :not_literal unless o.segments.size == 1 && o.segments[0].is_a?(Model::LiteralString)

spec/unit/pops/evaluator/literal_evaluator_spec.rb

@@ -17,6 +17,7 @@
     'a'       => 'a',
     'a::b'    => 'a::b',
     'Integer[1]' => [1],
+    'Integer[-1]' => [-1],
     'File' => "file",
 
     # special values
@@ -37,7 +38,16 @@
     expect(leval.literal(parser.parse_string('undef'))).to be_nil
   end
 
-  ['1+1', '[1,2, 1+2]', '{a=>1+1}', '"x$y"', '"x${y}z"', 'Integer[1-3]', 'Optional[[String]]'].each do |source|
+  [ '',
+    '1+1',
+    '[1,2, 1+2]',
+    '{a=>1+1}',
+    '"x$y"',
+    '"x${y}z"',
+    'Integer[1-3]',
+    'Integer[-1-3]',
+    'Optional[[String]]'
+  ].each do |source|
     it "throws :not_literal for non literal expression '#{source}'" do
       expect{leval.literal(parser.parse_string(source))}.to throw_symbol(:not_literal)
     end