[clang] [libc] Add invoke / invoke_result type traits (PR #65750)

[Clement Courbet via cfe-commits]

================
@@ -145,6 +145,110 @@ TEST(LlvmLibcTypeTraitsTest, integral_constant) {
   EXPECT_EQ((integral_constant<int, 4>::value), 4);
 }
 
+namespace invoke_detail {
+
+enum State { INIT = 0, A_APPLY_CALLED, B_APPLY_CALLED };
+
+struct A {
+  State state = INIT;
+  virtual ~A() {}
+  virtual void apply() { state = A_APPLY_CALLED; }
+};
+
+struct B : public A {
+  virtual ~B() {}
+  virtual void apply() { state = B_APPLY_CALLED; }
+};
+
+void free_function() {}
+int free_function_return_5() { return 5; }
+int free_function_passtrough(int value) { return value; }
+
+struct Delegate {
+  int (*ptr)(int) = &free_function_passtrough;
+};
+
+template <int tag> struct Tag {
+  static int value() { return tag; }
----------------
legrosbuffle wrote:

`static constexpr int value = tag;` ?

https://github.com/llvm/llvm-project/pull/65750