See the comment in Compiler<>::VisitCXXThisExpr.
We need to mark the InitList explicitly, so we later know what to refer
to when the init chain is active.
Unlike brace initialization, the parenthesized aggregate initialization
in C++20 does not extend the lifetime of a temporary object bound to a
reference in an aggreate. This can lead to dangling references:
```
struct A { const int& r; };
A a1(1); // well-formed, but results in a dangling reference.
```
With this patch, clang will diagnose this common dangling issues.
Fixes#101957
