[clang] [Analyzer][CFG] Correctly handle rebuilt default arg and default init expression (PR #117437)

[Hans Wennborg via cfe-commits]

zmodem wrote:

We're hitting an assert after this:

```
clang/lib/Analysis/CFG.cpp:822: void (anonymous namespace)::CFGBuilder::appendStmt(CFGBlock *, const Stmt *): Assertion `!isa<Expr>(S) || cast<Expr>(S)->IgnoreParens() == S' failed.
```

See https://crbug.com/394015869 for stack trace and reproducer. I'll revert for now to unbreak the build, and start a creduce to see if we can get a smaller repro.

https://github.com/llvm/llvm-project/pull/117437