fmt::ptr: Support function pointers (#2131)

Passing a function pointer to fmt::ptr results in:

 In file included from /home/mac/git/fmt/test/gmock/gmock.h:238,
                  from /home/mac/git/fmt/test/format-test.cc:31:
 .../fmt/test/format-test.cc: In member function ‘virtual void FormatterTest_FormatPointer_Test::TestBody()’:
 .../fmt/test/format-test.cc:1486:56: error: no matching function for call to ‘ptr(void (&)(int, double, std::__cxx11::string))’
              format("{}", fmt::ptr(function_pointer_test)));

with GCC and Clang. Let's add an overload to support that usage.

Unfortunately, MSVC would
consider the overload to be ambiguous for unknown reasons:

 D:\a\fmt\fmt\test\format-test.cc(1485,1): error C2668: 'fmt::v7::ptr': ambiguous call to overloaded function [D:\a\fmt\build\test\format-test.vcxproj]
 D:\a\fmt\fmt\include\fmt/format.h(3742,60): message : could be 'const void *fmt::v7::ptr<void,int,double,std::string>(T (__cdecl *)(int,double,std::string))' [D:\a\fmt\build\test\format-test.vcxproj]
           with
           [
               T=void
           ]
 D:\a\fmt\fmt\include\fmt/format.h(3735,42): message : or       'const void *fmt::v7::ptr<void(int,double,std::string)>(T (__cdecl *))' [D:\a\fmt\build\test\format-test.vcxproj]
           with
           [
               T=void (int,double,std::string)
           ]
 D:\a\fmt\fmt\test\format-test.cc(1486,1): message : while trying to match the argument list '(overloaded-function)' [D:\a\fmt\build\test\format-test.vcxproj]

but luckily this means that the overload is unnecessary in that case
anyway, so we can just make it conditional.

include/fmt/format.h

@@ -3739,6 +3739,13 @@ template <typename T> inline const void* ptr(const std::unique_ptr<T>& p) {
 template <typename T> inline 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...)) {
+  return detail::bit_cast<const void *>(fn);
+}
+#endif
 
 class bytes {
  private: