[clang] Attribute support `[[clang::musttail]]` in `ExprConstant.cpp` (work in progress) (PR #138477)

[Hana Dusíková via cfe-commits]

hanickadot wrote:

```c++
constexpr int deep_test(int remaining) {
	if (remaining == 0) {
		return 42;
	}
	MUST_TAIL return deep_test(remaining - 1);
}

constexpr int result = deep_test(200'000);
```

With debug build of clang:
```sh
clang++ -std=c++26 -c deep.cpp -fconstexpr-depth=1 -DMUST_TAIL="[[clang::musttail]]"

real	0m7.227s
user	0m7.199s
sys	0m0.019s
```

```sh
clang++ -std=c++26 -c deep.cpp -fconstexpr-depth=200000 -DMUST_TAIL=

Segmentation fault: 11
```


https://github.com/llvm/llvm-project/pull/138477