[Threading] Ensure Tests Reflect Disabled Threads

[lenary]

Summary

D137836 changed what llvm::get_physical_cores returns when threads are
disabled, to bring it inline with the other parts of Threading. It now
returns the value for "unknown" when threading is disabled.

This commit updates the tests (which are failing on some platforms), to
also reflect this change.

[DavidSpickett]

LGTM, thanks.

llvm/unittests/Support/Threading.cpp

 #include <atomic>
 #include <condition_variable>
 
 using namespace llvm;
 
 namespace {
 
 static bool isThreadingSupportedArchAndOS() {
+#if LLVM_ENABLE_THREADS
   Triple Host(Triple::normalize(sys::getProcessTriple()));
 
   // Initially this is only testing detection of the number of
   // physical cores, which is currently only supported/tested on
   // some systems.
   return (Host.isOSWindows() && llvm_is_multithreaded()) || Host.isOSDarwin() ||
          (Host.isX86() && Host.isOSLinux()) ||
          (Host.isOSLinux() && !Host.isAndroid()) ||
          (Host.isSystemZ() && Host.isOSzOS());
+#else
+  return false;
+#endif
 }
 
 TEST(Threading, PhysicalConcurrency) {
   auto Num = heavyweight_hardware_concurrency();
   // Since Num is unsigned this will also catch us trying to
   // return -1.
   ASSERT_LE(Num.compute_thread_count(),
             hardware_concurrency().compute_thread_count());