Check for atomic<double> intrinsics

On some architectures e.g. x86/32bit gcc decides to inline calls to
double atomic variables but clang does not and defers it to libatomic
therefore detect if clang can use built-ins for atomic<double> if not
then link libatomic, this helps building clangd for x86 on linux systems
with gcc runtime

Upstream-Status: Pending
Signed-off-by: Khem Raj <raj.khem@gmail.com>

Author: kraj

llvm/cmake/modules/CheckAtomic.cmake

@@ -30,10 +30,11 @@ function(check_working_cxx_atomics64 varname)
 #include <atomic>
 #include <cstdint>
 std::atomic<uint64_t> x (0);
+std::atomic<double> y (0);
 int main() {
   uint64_t i = x.load(std::memory_order_relaxed);
   (void)i;
-  return 0;
+  return int(y);
 }
 " ${varname})
   set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})