[toolchain] Implement lexing and parsing support for #543. (#693)

Lex [iuf][1-9][0-9]* as a new kind of "sized type literal" token. When
parsing that token, form a literal expression.

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

toolchain/parser/parser_impl.cpp

@@ -678,6 +678,9 @@ auto ParseTree::Parser::ParsePrimaryExpression() -> llvm::Optional<Node> {
     case TokenKind::IntegerLiteral():
     case TokenKind::RealLiteral():
     case TokenKind::StringLiteral():
+    case TokenKind::IntegerTypeLiteral():
+    case TokenKind::UnsignedIntegerTypeLiteral():
+    case TokenKind::FloatingPointTypeLiteral():
       kind = ParseNodeKind::Literal();
       break;