Add Cmake Check for atomic<double> intrinsic

[raj.khem]

Summary

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

[lebedev.ri]

Please upload all patches with full context (-U99999)

[raj.khem]

Can someone review this please?

llvm/cmake/modules/CheckAtomic.cmake

 
 function(check_working_cxx_atomics64 varname)
   set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
   set(CMAKE_REQUIRED_FLAGS "-std=c++11 ${CMAKE_REQUIRED_FLAGS}")
   CHECK_CXX_SOURCE_COMPILES("
 #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);
-  return 0;
+  return int(y);
 }
 " ${varname})
   set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
 endfunction(check_working_cxx_atomics64)
 
 
 # This isn't necessary on MSVC, so avoid command-line switch annoyance
 # by only running on GCC-like hosts.