libSyntax: teach parser to parse dictionary and array literals. (#12821)

This commit also adds ArrayExpr and DictionaryExpr to the libSyntax nodes
family. Also, it refactors the original parser code for these two
expressions to better fit to the design of SyntaxParsingContext.

This commit has also fixed two crashers.

utils/gyb_syntax_support/ExprNodes.py

@@ -19,6 +19,12 @@ EXPR_NODES = [
     Node('FunctionCallArgumentList', kind='SyntaxCollection',
          element='FunctionCallArgument'),
 
+    Node('ArrayElementList', kind='SyntaxCollection',
+         element='ArrayElement'),
+
+    Node('DictionaryElementList', kind='SyntaxCollection',
+         element='DictionaryElement'),
+
     # The try operator.
     # try foo()
     # try? foo()
@@ -89,6 +95,22 @@ EXPR_NODES = [
              Child('RightParen', kind='RightParenToken'),
          ]),
 
+    # Array literal, e.g. [1, 2, 3]
+    Node('ArrayExpr', kind='Expr',
+         children=[
+             Child('LeftSquare', kind='LeftSquareToken'),
+             Child('Elements', kind='ArrayElementList'),
+             Child('RightSquare', kind='RightSquareToken'),
+         ]),
+
+    # Dictionary literal, e.g. [1:1, 2:2, 3:3]
+    Node('DictionaryExpr', kind='Expr',
+         children=[
+             Child('LeftSquare', kind='LeftSquareToken'),
+             Child('Elements', kind='DictionaryElementList'),
+             Child('RightSquare', kind='RightSquareToken'),
+         ]),
+
     # function-call-argument -> label? ':'? expression ','?
     Node('FunctionCallArgument', kind='Syntax',
          children=[
@@ -101,6 +123,22 @@ EXPR_NODES = [
                    is_optional=True),
          ]),
 
+    # element inside an array expression: expression ','?
+    Node('ArrayElement', kind='Syntax',
+         children=[
+             Child('Expression', kind='Expr'),
+             Child('TrailingComma', kind='CommaToken', is_optional=True),
+         ]),
+
+    # element inside an array expression: expression ','?
+    Node('DictionaryElement', kind='Syntax',
+         children=[
+             Child('KeyExpression', kind='Expr'),
+             Child('Colon', kind='ColonToken'),
+             Child('ValueExpression', kind='Expr'),
+             Child('TrailingComma', kind='CommaToken', is_optional=True),
+         ]),
+
     # An integer literal.
     # 3
     # +3_400