Drop support for named tuple fields (#886)

Rationale: Based on the status of #478 and #505, Carbon won't have this feature for a while, and it will be simpler not to support it on spec in the meantime.

executable_semantics/testdata/basic_syntax/record.carbon

@@ -12,7 +12,7 @@
 package ExecutableSemanticsTest api;
 
 fn main() -> i32 {
-  var t2: (.x = i32, .y = i32) = (.x = 2, .y = 5);
+  var t2: {.x: i32, .y: i32} = {.x = 2, .y = 5};
   t2.y = 3;
   return t2.y - t2.x - 1; // 3 - 2 - 1
 }