[CUDA] Define __CUDACC__ before standard library headers

libstdc++ since version 7 when GNU extensions are enabled (e.g. -std=gnu++11)
use it to avoid defining overloads using `__float128`.  This fixes compiling
with GNU extensions failing due to `__float128` being used.

Discovered at https://gitlab.kitware.com/cmake/cmake/-/merge_requests/4442#note_737136.

Differential Revision: https://reviews.llvm.org/D78392

clang/lib/Headers/__clang_cuda_runtime_wrapper.h

@@ -31,11 +31,17 @@
 // Include some forward declares that must come before cmath.
 #include <__clang_cuda_math_forward_declares.h>
 
+// Define __CUDACC__ early as libstdc++ standard headers with GNU extensions
+// enabled depend on it to avoid using __float128, which is unsupported in
+// CUDA.
+#define __CUDACC__
+
 // Include some standard headers to avoid CUDA headers including them
 // while some required macros (like __THROW) are in a weird state.
 #include <cmath>
 #include <cstdlib>
 #include <stdlib.h>
+#undef __CUDACC__
 
 // Preserve common macros that will be changed below by us or by CUDA
 // headers.