[clang] [Clang] Implement the core language parts of P2786 - Trivial relocation (PR #127636)

[via cfe-commits]

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+// RUN: %clang_cc1 -std=c++2c -verify %s
+
+class Trivial {};
+struct NonRelocatable {
+    ~NonRelocatable();
+};
+static NonRelocatable NonRelocatable_g;
+
+class A trivially_relocatable_if_eligible {};
+class B trivially_relocatable_if_eligible : Trivial{};
+class C trivially_relocatable_if_eligible {
+    int a;
+    void* b;
+    int c[3];
+    Trivial d[3];
+    NonRelocatable& e = NonRelocatable_g;
+};
+class D trivially_relocatable_if_eligible : Trivial {};
+class E trivially_relocatable_if_eligible : virtual Trivial {};
+
+class F trivially_relocatable_if_eligible : NonRelocatable {};
+
+class I trivially_relocatable_if_eligible {
+    NonRelocatable a;
+    NonRelocatable b[1];
+    const NonRelocatable c;
+    const NonRelocatable d[1];
+};
+
+class J trivially_relocatable_if_eligible:  virtual Trivial, NonRelocatable {
+    NonRelocatable a;
+};
+
+class G trivially_relocatable_if_eligible {
+    G(G&&);
+};
+
+class H trivially_relocatable_if_eligible {
+    ~H();
+};
+
+struct Incomplete; // expected-note {{forward declaration of 'Incomplete'}}
+static_assert(__builtin_is_cpp_trivially_relocatable(Incomplete));  // expected-error {{incomplete type 'Incomplete' used in type trait expression}}
+static_assert(__builtin_is_cpp_trivially_relocatable(Trivial));
+static_assert(__builtin_is_cpp_trivially_relocatable(G));
+static_assert(__builtin_is_cpp_trivially_relocatable(H));
+static_assert(__builtin_is_cpp_trivially_relocatable(int));
+static_assert(__builtin_is_cpp_trivially_relocatable(void*));
+static_assert(!__builtin_is_cpp_trivially_relocatable(int&));
+static_assert(!__builtin_is_cpp_trivially_relocatable(Trivial&));
+static_assert(__builtin_is_cpp_trivially_relocatable(const Trivial));
+static_assert(__builtin_is_cpp_trivially_relocatable(Trivial[1]));
----------------
Sirraide wrote:

Hmm, does this not do what I think it does?
```c++
int n = 4;
static_assert(__builtin_is_cpp_trivially_relocatable(int[n]));
```

https://github.com/llvm/llvm-project/pull/127636