Adds `TemplateTypeChecker` operation to retrieve the `Symbol` of a
`TmplAstVariable` or `TmplAstReference` in a template.
Sometimes we need to traverse an intermediate variable declaration to arrive at
the correct `ts.Symbol`. For example, loop variables are declared using an intermediate:
```
<div *ngFor="let user of users">
{{user.name}}
</div>
```
Getting the symbol of user here (from the expression) is tricky, because the TCB looks like:
```
var _t0 = ...; // type of NgForOf
var _t1: any; // context of embedded view for NgForOf structural directive
if (NgForOf.ngTemplateContextGuard(_t0, _t1)) {
// _t1 is now NgForOfContext<...>
var _t2 = _t1.$implicit; // let user = '$implicit'
_t2.name; // user.name expression
}
```
Just getting the `ts.Expression` for the `AST` node `PropRead(ImplicitReceiver, 'user')`
via the sourcemaps will yield the `_t2` expression. This function recognizes that `_t2`
is a variable declared locally in the TCB, and actually fetch the `ts.Symbol` of its initializer.
These special handlings show the versatility of the `Symbol`
interface defined in the API. With this, when we encounter a template variable,
we can provide the declaration node, as well as specific information
about the variable instance, such as the `ts.Type` and `ts.Symbol`.
PR Close#38618
19598b47ca