Infer element types for array literals and NilClass for nil literals

Array literal chains (`[1, 2, 3]`) previously resolved to a bare
`Array` with no element type. Infer each child's type and attach it
as the Array's generic parameter, falling back to plain `Array` when
empty or undefined.

`simplify_literals` left the `nil` pseudo-type tag as-is instead of
converting it to `NilClass` like other literals are converted to their
class names. This also surfaced two previously-pending specs
(NilClass/nil conformance, and passing a NilClass value to a `nil`
parameter) that now pass.

Author: apiology

lib/solargraph/complex_type/unique_type.rb

@@ -103,6 +103,7 @@ def to_s
       # @return [self]
       def simplify_literals
         transform do |t|
+          next t.recreate(new_name: 'NilClass') if t.nil_type?
           next t unless t.literal?
           t.recreate(new_name: t.non_literal_name)
         end
@@ -308,7 +309,7 @@ def to_rbs
           'untyped'
         elsif name == 'Boolean'
           'bool'
-        elsif name.downcase == 'nil'
+        elsif name.downcase == 'nil' || name == 'NilClass'
           'nil'
         elsif name == GENERIC_TAG_NAME
           all_params.first&.name

spec/pin/base_variable_spec.rb

@@ -44,7 +44,7 @@ def bar
     expect(type.tags).to eq('1, nil')
     expect(type.simple_tags).to eq('Integer, NilClass')
     expect(type.to_rbs).to eq('(1 | nil)')
-    expect(type.simplify_literals.to_rbs).to eq('(::Integer | ::NilClass)')
+    expect(type.simplify_literals.to_rbs).to eq('(::Integer | nil)')
   end
 
   it "understands proc kwarg parameters aren't affected by @type" do