[FIX] compiler: t-model supports radio group in t-foreach

Have a radio group defined inside a t-foreach:
```xml
  <t t-foreach="values" t-as="val" t-key="val">
    <input name="radiogroup" t-att-value="val" t-model="state.radioGroup" />
  </t>
```

Before this commit the algorithm that set the "checked" attribute on the current active
radio button according to the state did not support having a dynamic value (`t-att-value`)

After this commit, this use case works as we go look in the dynamic attributes too.

src/compiler/code_generator.ts

@@ -632,7 +632,15 @@ export class CodeGenerator {
 
       let idx: number;
       if (specialInitTargetAttr) {
-        idx = block!.insertData(`${fullExpression} === '${attrs[targetAttr]}'`, "attr");
+        let targetExpr = targetAttr in attrs && `'${attrs[targetAttr]}'`;
+        if (!targetExpr && ast.attrs) {
+          // look at the dynamic attribute counterpart
+          const dynamicTgExpr = ast.attrs[`t-att-${targetAttr}`];
+          if (dynamicTgExpr) {
+            targetExpr = compileExpr(dynamicTgExpr);
+          }
+        }
+        idx = block!.insertData(`${fullExpression} === ${targetExpr}`, "attr");
         attrs[`block-attribute-${idx}`] = specialInitTargetAttr;
       } else if (hasDynamicChildren) {
         const bValueId = generateId("bValue");