Update precedence rules to match design. (#3081)

- Only allow assignment at the top level in an expression statement.
- Allow both negation and complement as subexpressions of both bitwise
  and numeric operators.
- Remove parsing support for postincrement and postdecrement.
- Add parsing support for `as` operator.
- Use the same ambient precedence for types and non-type expressions.

---------

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

toolchain/parser/parser_handle_statement.cpp

@@ -46,7 +46,7 @@ auto ParserHandleStatement(ParserContext& context) -> void {
     }
     default: {
       context.PushState(ParserState::ExpressionStatementFinish);
-      context.PushState(ParserState::Expression);
+      context.PushStateForExpression(PrecedenceGroup::ForExpressionStatement());
       break;
     }
   }