fix fmt::get for some GCC versions and legacy Clang

fixes fmtlib#2140

- some GCC versions decay function pointers to `const void*`, exactly like
  MSVC does
- legacy Clang (prior to 7.0) treats function pointers also as `const T*`
  pointers, but unable to convert them

include/fmt/format.h

@@ -3833,21 +3833,27 @@ FMT_CONSTEXPR void advance_to(
     auto s = fmt::format("{}", fmt::ptr(p));
   \endrst
  */
-template <typename T> inline const void* ptr(const T* p) { return p; }
-template <typename T> inline const void* ptr(const std::unique_ptr<T>& p) {
+template <typename T>
+#if FMT_CLANG_VERSION && FMT_CLANG_VERSION < 700
+enable_if_t<!std::is_function<T>::value, const void*>
+#else
+const void*
+#endif
+ptr(const T* p) {
+  return p;
+}
+template <typename T> const void* ptr(const std::unique_ptr<T>& p) {
   return p.get();
 }
-template <typename T> inline const void* ptr(const std::shared_ptr<T>& p) {
+template <typename T> const void* ptr(const std::shared_ptr<T>& p) {
   return p.get();
 }
-#if !FMT_MSC_VER
-// MSVC lets function pointers decay to void pointers, so this
-// overload is unnecessary.
 template <typename T, typename... Args>
-inline const void* ptr(T (*fn)(Args...)) {
+auto ptr(T (*fn)(Args...))
+    -> enable_if_t<!std::is_convertible<decltype(fn), const void*>(),
+                   const void*> {
   return detail::bit_cast<const void*>(fn);
 }
-#endif
 
 class bytes {
  private: