[backport][Clang] Defer the instantiation of explicit-specifier until constraint checking completes · Pull Request !81 · openEuler/llvm-project

Backport upstream commit
https://github.com/llvm/llvm-project/commit/128b3b61fe6768c724975fd1df2be0abec848cf6
to fix an build failure.

CentOS already fixed this issue in release clang-17.0.6-6.el9.

Reproduce:
```c++
// test.cpp
template<class X>
constexpr X failure() {
    return nullptr;
}

struct T {
    template<class X>
        requires false
    constexpr explicit(failure<X>()) T(X) { }

T(int) { }
};

static T t(5);
```

On openEuler 2403,
```sh
[root@Euler2403 BUILD]# rpm -q clang gcc
clang-17.0.6-10.oe2403.x86_64
gcc-12.3.1-18.oe2403.x86_64
```
Building with `clang test.cpp -std=c++20 -O2 -fsyntax-only`，it failed with:
```sh
test.cpp:3:12: error: cannot initialize return object of type 'int' with an rvalue of type 'std::nullptr_t'
    3 |     return nullptr;
      |            ^~~~~~~
test.cpp:9:24: note: in instantiation of function template specialization 'failure<int>' requested here
    9 |     constexpr explicit(failure<X>()) T(X) { }
      |                        ^
test.cpp:14:10: note: while substituting deduced template arguments into function template 'T' [with X = int]
   14 | static T t(5);
      |          ^
1 error generated.
```

building with `gcc test.cpp -std=c++20 -O2 -fsyntax-only`, it succeeded.

With this patch applied, clang build succeed.

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