[Sema] -Wpointer-bool-conversion: suppress lambda function pointer conversion diagnostic during instantiation (#83497)

I have seen two internal pieces of code that uses a template type
parameter to accept any callable type (function pointer, std::function,
closure type, etc). The diagnostic added in #83152 would require
adaptation to the template, which is difficult and also seems
unnecessary. Example:

```cpp
template <typename... Ts>
static bool IsFalse(const Ts&...) { return false; }

template <typename T, typename... Ts,
          typename = typename std::enable_if<std::is_constructible<bool, const T&>::value>::type>
static bool IsFalse(const T& p, const Ts&...) {
  return p ? false : true;
}

template <typename... Args>
void Init(Args&&... args) {
  if (IsFalse(absl::implicit_cast<const typename std::decay<Args>::type&>(
              args)...)) {
    // A callable object convertible to false is either a null pointer or a
    // null functor (e.g., a default-constructed std::function).
    empty_ = true;
  } else {
    empty_ = false;
    new (&factory_) Factory(std::forward<Args>(args)...);
  }
}
```

Author: MaskRay

clang/lib/Sema/SemaChecking.cpp

@@ -16588,13 +16588,16 @@ void Sema::DiagnoseAlwaysNonNullPointer(Expr *E,
   }
 
   // Complain if we are converting a lambda expression to a boolean value
-  if (const auto *MCallExpr = dyn_cast<CXXMemberCallExpr>(E)) {
-    if (const auto *MRecordDecl = MCallExpr->getRecordDecl();
-        MRecordDecl && MRecordDecl->isLambda()) {
-      Diag(E->getExprLoc(), diag::warn_impcast_pointer_to_bool)
-          << /*LambdaPointerConversionOperatorType=*/3
-          << MRecordDecl->getSourceRange() << Range << IsEqual;
-      return;
+  // outside of instantiation.
+  if (!inTemplateInstantiation()) {
+    if (const auto *MCallExpr = dyn_cast<CXXMemberCallExpr>(E)) {
+      if (const auto *MRecordDecl = MCallExpr->getRecordDecl();
+          MRecordDecl && MRecordDecl->isLambda()) {
+        Diag(E->getExprLoc(), diag::warn_impcast_pointer_to_bool)
+            << /*LambdaPointerConversionOperatorType=*/3
+            << MRecordDecl->getSourceRange() << Range << IsEqual;
+        return;
+      }
     }
   }
 

clang/test/SemaCXX/warn-bool-conversion.cpp

@@ -92,6 +92,21 @@ void foo() {
   bool is_true = [](){ return true; };
   // expected-warning@-1{{address of lambda function pointer conversion operator will always evaluate to 'true'}}
 }
+
+template <typename... Ts>
+static bool IsFalse(const Ts&...) { return false; }
+template <typename T>
+static bool IsFalse(const T& p) {
+  bool b;
+  b = f7; // expected-warning {{address of lambda function pointer conversion operator will always evaluate to 'true'}}
+  // Intentionally not warned on because p could be a lambda type in one
+  // instantiation, but a pointer type in another.
+  return p ? false : true;
+}
+
+bool use_instantiation() {
+  return IsFalse([]() { return 0; });
+}
 #endif
 
 void bar() {