add tests to cover unsupported cases

Author: a-tarasyuk

clang/test/SemaCXX/ms-constexpr-new.cpp

@@ -12,5 +12,12 @@ namespace std {
   }
 }
 
-constexpr bool check_construct_at() { int x; return *std::construct_at(&x, 42) == 42; }
-static_assert(check_construct_at());
+constexpr bool check_std_construct_at() { int x; return *std::construct_at(&x, 42) == 42; }
+static_assert(check_std_construct_at());
+
+constexpr int* construct_at(int* p, int v) { [[msvc::constexpr]] return ::new (p) int(v); } // unsupported-error {{constexpr function never produces a constant expression}} \
+                                                                                            // unsupported-warning {{unknown attribute 'constexpr' ignored}} \
+                                                                                            // unsupported-note 2{{this placement new expression is not supported in constant expressions before C++2c}}
+constexpr bool check_construct_at() { int x; return *construct_at(&x, 42) == 42; }          // unsupported-note {{in call to 'construct_at(&x, 42)'}}
+static_assert(check_construct_at());                                                        // unsupported-error {{static assertion expression is not an integral constant expression}}\
+                                                                                            // unsupported-note {{in call to 'check_construct_at()'}}