[clang] [clang] Add test for CWG3129 (PR #206335)

[Jan Schultke via cfe-commits]

eisenwave wrote:

Very weird. The CI failures is because it's printing
```
1.18973149535723176508575932662800702E+4932
```
as the maximum for `long double` in CI but the test expectations is
```
1.18973149535723176502E+4932
```
which did work for me locally. OH well ...

https://github.com/llvm/llvm-project/pull/206335