[CUDA][Windows] Last fix for the clang Bug 38811 "Clang fails to compile with CUDA...

[CUDA][Windows] Last fix for the clang Bug 38811 "Clang fails to compile with CUDA-9.x on Windows" (https://bugs.llvm.org/show_bug.cgi?id=38811).

[IMPORTANT]
With that last fix, CUDA has just started being compiling by clang on Windows after nearly a year and two clang’s major releases (7 and 8).
As long as the last LLVM release, in which clang was compiling CUDA on Windows successfully, was 6.0.1, this fix and two previous have to be included into upcoming 7.1.0 and 8.0.1 releases.

[How to repro]
clang++.exe -x cuda "c:\ProgramData\NVIDIA Corporation\CUDA Samples\v9.0\0_Simple\simplePrintf\simplePrintf.cu" -I"c:\ProgramData\NVIDIA Corporation\CUDA Samples\v9.0\common\inc" --cuda-gpu-arch=sm_50 --cuda-path="C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v9.0" -L"c:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v9.0\lib\x64" -lcudart.lib  -v

[Output]
In file included from C:\GIT\LLVM\trunk-for-submits\llvm-64-release-vs2017-15.9.9\dist\lib\clang\9.0.0\include\__clang_cuda_runtime_wrapper.h:327:
C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v9.0/include\crt/math_functions.hpp:390:11: error: no matching function for call to '__isinfl'
  return (__isinfl(a) != 0);
          ^~~~~~~~
C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v9.0/include\crt/math_functions.hpp:2662:14: note: candidate function not viable: call to __host__ function from __device__ function
__func__(int __isinfl(long double a))
             ^
In file included from <built-in>:1:
In file included from C:\GIT\LLVM\trunk-for-submits\llvm-64-release-vs2017-15.9.9\dist\lib\clang\9.0.0\include\__clang_cuda_runtime_wrapper.h:327:
C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v9.0/include\crt/math_functions.hpp:438:11: error: no matching function for call to '__isnanl'
  return (__isnanl(a) != 0);
          ^~~~~~~~
C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v9.0/include\crt/math_functions.hpp:2672:14: note: candidate function not viable: call to __host__ function from __device__ function
__func__(int __isnanl(long double a))
             ^
In file included from <built-in>:1:
In file included from C:\GIT\LLVM\trunk-for-submits\llvm-64-release-vs2017-15.9.9\dist\lib\clang\9.0.0\include\__clang_cuda_runtime_wrapper.h:327:
C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v9.0/include\crt/math_functions.hpp:486:11: error: no matching function for call to '__finitel'
  return (__finitel(a) != 0);
          ^~~~~~~~~
C:\Program Files\NVIDIA GPU Computing Toolkit\CUDA\v9.0/include\crt/math_functions.hpp:2652:14: note: candidate function not viable: call to __host__ function from __device__ function
__func__(int __finitel(long double a))
             ^
3 errors generated when compiling for sm_50.

[Solution]
Add missing long double device functions' declarations. Provide only declarations to prevent any use of long double on the device side, because CUDA does not support long double on the device side.

[Testing]
{Windows 10, Ubuntu 16.04.5}/{Visual C++ 2017 15.9.9, gcc+ 5.4.0}/CUDA {8.0, 9.0, 9.1, 9.2, 10.0, 10.1}

Reviewed by: Artem Belevich

Differential Revision: http://reviews.llvm.org/D60220

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@357779 91177308-0d34-0410-b5e6-96231b3b80d8