[clang] [Clang] Implement CWG2517 Useless restriction on use of parameter in constraint-expression (PR #132919)

[Imad Aldij via cfe-commits]

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@@ -37,13 +37,6 @@ namespace std_example {
   static_assert(D<T1>);
   template<D T> struct D_check {}; // expected-note{{because 'short' does not satisfy 'D'}}
   using dc1 = D_check<short>; // expected-error{{constraints not satisfied for class template 'D_check' [with T = short]}}
-
-  template<typename T>
-  concept C2 = requires (T a) {
-      requires sizeof(a) == 4; // OK
-      requires a == 0; // expected-note{{because 'a == 0' would be invalid: constraint variable 'a' cannot be used in an evaluated context}}
-    };
-  static_assert(C2<int>); // expected-note{{because 'int' does not satisfy 'C2'}} expected-error{{static assertion failed}}
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imdj wrote:

If we keep the tests and only remove the expected comments, this results in:
```
error: 'expected-error' diagnostics seen but not expected: 
  File [...]/nested-requirement.cpp Line 44: substitution into constraint expression resulted in a non-constant expression
  File [...]/nested-requirement.cpp Line 46: static assertion failed
error: 'expected-note' diagnostics seen but not expected: 
  File [...]/nested-requirement.cpp Line 44: while checking the satisfaction of nested requirement requested here
  File [...]/nested-requirement.cpp Line 44: in instantiation of requirement here
  File [...]/nested-requirement.cpp Line 44: while checking the satisfaction of nested requirement requested here
  File [...]/nested-requirement.cpp Line 42: while substituting template arguments into constraint expression here
  File [...]/nested-requirement.cpp Line 46: while checking the satisfaction of concept 'C2<int>' requested here
  File [...]/nested-requirement.cpp Line 44: function parameter 'a' with unknown value cannot be used in a constant expression
  File [...]/nested-requirement.cpp Line 42: declared here
  File [...]/nested-requirement.cpp Line 46: because 'int' does not satisfy 'C2'

```

Should I keep the tests and update the expected failures instead?

https://github.com/llvm/llvm-project/pull/132919