[compiler-rt] [HWASan] leave BufferedStackTrace uninit (#102247)

Otherwise we have to memset 2040 bytes (255 * 8) for each call to a
malloc-like function.

This caused noticable slowdown on AOSP.

Author: fmayer

compiler-rt/lib/hwasan/hwasan.h

@@ -139,14 +139,14 @@ void hwasan_free(void *ptr, StackTrace *stack);
 void InstallAtExitHandler();
 
 #define GET_MALLOC_STACK_TRACE                                            \
-  BufferedStackTrace stack;                                               \
+  UNINITIALIZED BufferedStackTrace stack;                                 \
   if (hwasan_inited)                                                      \
     stack.Unwind(StackTrace::GetCurrentPc(), GET_CURRENT_FRAME(),         \
                  nullptr, common_flags()->fast_unwind_on_malloc,          \
                  common_flags()->malloc_context_size)
 
 #define GET_FATAL_STACK_TRACE_PC_BP(pc, bp)              \
-  BufferedStackTrace stack;                              \
+  UNINITIALIZED BufferedStackTrace stack;                \
   if (hwasan_inited)                                     \
     stack.Unwind(pc, bp, nullptr, common_flags()->fast_unwind_on_fatal)