Still not working for me:

$ sudo dnf upgrade --enablerepo=updates-testing --advisory=FEDORA-2019-3521ab1e66
Last metadata expiration check: 0:08:45 ago on Sat 09 Mar 2019 08:58:36 AM CET.
No security updates needed, but 99 updates available
Dependencies resolved.
Nothing to do.
Complete!

$ cat isnan.c
#include <math.h>

int main(void)
{
  double foo = 1.0;

  if (isnan(foo))
    return 1;
  return 0;
}

$ clang -Wdouble-promotion -Wall -o isnan isnan.c
isnan.c:7:13: warning: implicit conversion increases floating-point precision: 'double' to 'long double' [-Wdouble-promotion]
  if (isnan(foo))
      ~~~~~~^~~~
/usr/include/math.h:928:46: note: expanded from macro 'isnan'
#  define isnan(x) __MATH_TG ((x), __isnan, (x))
                   ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
/usr/include/math.h:846:16: note: expanded from macro '__MATH_TG'
   : FUNC ## l ARGS)
     ~~~~~~~~~ ^~~~
1 warning generated.

Works great! LGTM! =)

After seeing besser82's positive karma i double-checked and I'm pretty sure I saw no improvement with this very update:

$ dnf info clang --installed
Installed Packages
Name         : clang
Version      : 7.0.1
Release      : 5.fc29
Architecture : x86_64
Size         : 1.3 M
Source       : clang-7.0.1-5.fc29.src.rpm
Repository   : @System
From repo    : updates-testing
Summary      : A C language family front-end for LLVM
URL          : http://llvm.org
License      : NCSA
Description  : clang: noun
             :     1. A loud, resonant, metallic sound.
             :     2. The strident call of a crane or goose.
             :     3. C-language family front-end toolkit.
             : 
             : The goal of the Clang project is to create a new C, C++, Objective C
             : and Objective C++ front-end for the LLVM compiler. Its tools are built
             : as libraries and designed to be loosely-coupled and extensible.

This update has been unpushed.

I bundled the patch but fogot to apply it, thanks@dridi for spotting that.