Remove restriction on optional nesting

We previously disallowed OptionalMemberExpression inside a normal 
MemberExpression, eg `(a?.b).c`. The new representation handles this case 
correctly so we can remove the restriction.

compiler/forget/src/HIR/BuildHIR.ts

@@ -2113,32 +2113,6 @@ function lowerMemberExpression(
   const object =
     loweredObject ?? lowerExpressionToTemporary(builder, objectNode);
 
-  if (
-    objectNode.isOptionalMemberExpression() &&
-    !expr.isOptionalMemberExpression()
-  ) {
-    // Babel's `isOptionalMemberExpression` indicates whether this property load itself
-    // is conditional (i.e. within an "optional chain"). This is different from the
-    // `optional` property, which is only true for property loads at the start of an
-    // optional chain. e.g. `a.b?.c.d` decomposes into
-    //   [0] MemberExpr          a.b;   // not in an optional chain
-    //   [1] OptionalMemberExpr [0]?.c
-    //   [2] OptionalMemberExpr [1].c
-    //   [3] OptionalMemberExpr [2].d
-    // We currently do not handle non-conditional loads from an optional memberexpr
-    // e.g. `(a?.b).c`
-    // See error.nonoptional-load-from-optional-memberexpr test fixture for details
-    builder.errors.push({
-      reason: `(BuildHIR::lowerMemberExpression) Handle optional chaining for non-optional member expr.`,
-      severity: ErrorSeverity.Todo,
-      nodePath: propertyNode,
-    });
-    return {
-      object,
-      property: propertyNode.toString(),
-      value: { kind: "UnsupportedNode", node: exprNode, loc: exprLoc },
-    };
-  }
   if (!expr.node.computed) {
     if (!propertyNode.isIdentifier()) {
       builder.errors.push({

compiler/forget/src/__tests__/fixtures/compiler/nonoptional-load-from-optional-memberexpr.expect.md

@@ -13,18 +13,17 @@ function Component(props) {
 
 ```
 
+## Code
 
-## Error
+```javascript
+// Note that `a?.b.c` is semantically different from `(a?.b).c`
+// Here, 'props?.a` is an optional chain, and `.b` is an unconditional load
+// (nullthrows if a is nullish)
+
+function Component(props) {
+  const x = (props?.a).b;
+  return x;
+}
 
 ```
-[ReactForget] TodoError: (BuildHIR::lowerMemberExpression) Handle optional chaining for non-optional member expr.
-  4 |
-  5 | function Component(props) {
-> 6 |   let x = (props?.a).b;
-    |                      ^
-  7 |   return x;
-  8 | }
-  9 |
-```
-