[Support] Properly zero initialize CPU set when querying affinity (#142924)

This commit resolves a potential issue of working with uninitialized
memory when querying the CPU's affinity. The man page of
`sched_getaffinity` does not guarantee that the memory will be fully
overwritten, so this change should ensure that issues are avoided.

llvm/lib/Support/Unix/Threading.inc

@@ -318,6 +318,7 @@ static int computeHostNumHardwareThreads() {
     return CPU_COUNT(&mask);
 #elif defined(__linux__)
   cpu_set_t Set;
+  CPU_ZERO(&Set);
   if (sched_getaffinity(0, sizeof(Set), &Set) == 0)
     return CPU_COUNT(&Set);
 #endif