libSyntax: parse codeblock syntax node. (#12771)

This commit teaches parser to generate code block syntax node. As a support for this, 
SyntaxParsingContext can be created by a single syntax kind, indicating the whole context 
should be parsed into a node of that given syntax. Another change is to bridge created syntax 
node with the given context kind. For instance, if a statement context results into an expression 
node, the expression node will be bridged to a statement by wrapping it with a ExpressionStmt 
node.

lib/Syntax/RawSyntax.cpp

@@ -29,6 +29,7 @@ static bool isTrivialSyntaxKind(SyntaxKind Kind) {
   case SyntaxKind::SourceFile:
   case SyntaxKind::TopLevelCodeDecl:
   case SyntaxKind::ExpressionStmt:
+  case SyntaxKind::DeclarationStmt:
     return true;
   default:
     return false;