Add a declaration for __cpuid for the IntelLLVM compiler.

__cpuid is declared in intrin.h, but is "excluded" on non-Windows
platforms.
We add this declaration to compensate.

Fixes #1358

absl/random/internal/randen_detect.cc

@@ -45,6 +45,10 @@
 #if defined(ABSL_INTERNAL_USE_X86_CPUID)
 #if defined(_WIN32) || defined(_WIN64)
 #include <intrin.h>  // NOLINT(build/include_order)
+#elif ABSL_HAVE_BUILTIN(__cpuid)
+// MSVC-equivalent __cpuid intrinsic declaration for clang-like compilers
+// for non-Windows build environments.
+void __cpuid(int[4], int);
 #else
 // MSVC-equivalent __cpuid intrinsic function.
 static void __cpuid(int cpu_info[4], int info_type) {