[sanitizer] Remove GetCurrentThread nullness checks from Allocate

The nullness check is unreachable.

* For the main thead and pthread_create created threads, the `*Allocate` functions must be called after `*_current_thread` is set.
set.
* For threads created by Linux's `clone`, static TLS is either reused or
  set to a new value (CLONE_SETTLS).

Make this change for asan/msan and possibly extend the change to other
sanitizers. (asan supports many platforms and I am not 100% certain that
all platforms have the property.)

Pull Request: llvm#102828

compiler-rt/lib/asan/asan_allocator.cpp

@@ -576,15 +576,8 @@ struct Allocator {
     }
 
     AsanThread *t = GetCurrentThread();
-    void *allocated;
-    if (t) {
-      AllocatorCache *cache = GetAllocatorCache(&t->malloc_storage());
-      allocated = allocator.Allocate(cache, needed_size, 8);
-    } else {
-      SpinMutexLock l(&fallback_mutex);
-      AllocatorCache *cache = &fallback_allocator_cache;
-      allocated = allocator.Allocate(cache, needed_size, 8);
-    }
+    void *allocated = allocator.Allocate(
+        GetAllocatorCache(&t->malloc_storage()), needed_size, 8);
     if (UNLIKELY(!allocated)) {
       SetAllocatorOutOfMemory();
       if (AllocatorMayReturnNull())

compiler-rt/lib/msan/msan_allocator.cpp

@@ -199,15 +199,8 @@ static void *MsanAllocate(BufferedStackTrace *stack, uptr size, uptr alignment,
     ReportRssLimitExceeded(stack);
   }
   MsanThread *t = GetCurrentThread();
-  void *allocated;
-  if (t) {
-    AllocatorCache *cache = GetAllocatorCache(&t->malloc_storage());
-    allocated = allocator.Allocate(cache, size, alignment);
-  } else {
-    SpinMutexLock l(&fallback_mutex);
-    AllocatorCache *cache = &fallback_allocator_cache;
-    allocated = allocator.Allocate(cache, size, alignment);
-  }
+  void *allocated = allocator.Allocate(GetAllocatorCache(&t->malloc_storage()),
+                                       size, alignment);
   if (UNLIKELY(!allocated)) {
     SetAllocatorOutOfMemory();
     if (AllocatorMayReturnNull())