sanitizer: fix GCC warnings

[marxin]

Summary

Fixes the following warnings:

sanitizer_stack_store.cpp:192:17: warning: ‘diff’ may be used uninitialized [-Wmaybe-uninitialized]
ubsan/ubsan_diag.h:193:21: warning: ‘CheckKindStr’ may be used uninitialized [-Wmaybe-uninitialized]
tsan/tsan_rtl.h:693:3: warning: ‘evp’ may be used uninitialized [-Wmaybe-uninitialized]
sanitizer_posix.cpp:163:27: warning: ‘flags’ may be used uninitialized [-Wmaybe-uninitialized]

compiler-rt/lib/sanitizer_common/sanitizer_posix.cpp

 fd_t OpenFile(const char *filename, FileAccessMode mode, error_t *errno_p) {
   if (ShouldMockFailureToOpen(filename))
     return kInvalidFd;
   int flags;
   switch (mode) {
     case RdOnly: flags = O_RDONLY; break;
     case WrOnly: flags = O_WRONLY | O_CREAT | O_TRUNC; break;
     case RdWr: flags = O_RDWR | O_CREAT; break;
+    default: Die();
   }
   fd_t res = internal_open(filename, flags, 0660);
   if (internal_iserror(res, errno_p))
     return kInvalidFd;
   return ReserveStandardFds(res);
 }
 
 void CloseFile(fd_t fd) {

compiler-rt/lib/sanitizer_common/sanitizer_stack_store.cpp

bool operator==(const SLeb128Decoder &other) const {
     return begin == other.begin;
   }
 
   bool operator!=(const SLeb128Decoder &other) const {
     return begin != other.begin;
   }
 
   uptr operator*() {
-    sptr diff;
+    sptr diff = 0;
     begin = DecodeSLEB128(begin, end, &diff);
     previous += diff;
     return previous;
   }
   SLeb128Decoder &operator++() { return *this; }
 
   SLeb128Decoder operator++(int) { return *this; }
 

compiler-rt/lib/tsan/rtl/tsan_rtl.h

 template <typename EventT>
 ALWAYS_INLINE void TraceRelease(ThreadState *thr, EventT *evp) {
   DCHECK_LE(evp + 1, &thr->tctx->trace.parts.Back()->events[TracePart::kSize]);
   atomic_store_relaxed(&thr->trace_pos, (uptr)(evp + 1));
 }
 
 template <typename EventT>
 void TraceEvent(ThreadState *thr, EventT ev) {
-  EventT *evp;
+  EventT *evp = nullptr;
   if (!TraceAcquire(thr, &evp)) {
     TraceSwitchPart(thr);
     UNUSED bool res = TraceAcquire(thr, &evp);
     DCHECK(res);
   }
   *evp = ev;
   TraceRelease(thr, evp);
 }

compiler-rt/lib/ubsan/ubsan_handlers_cxx.cpp

void __ubsan_handle_cfi_bad_type(CFICheckFailData *Data, ValueHandle Vtable,
   case CFITCK_UnrelatedCast:
     CheckKindStr = "cast to unrelated type";
     break;
   case CFITCK_VMFCall:
     CheckKindStr = "virtual pointer to member function call";
     break;
   case CFITCK_ICall:
   case CFITCK_NVMFCall:
+  default:
     Die();
   }
 
   Diag(Loc, DL_Error, ET,
        "control flow integrity check for type %0 failed during "
        "%1 (vtable address %2)")
       << Data->Type << CheckKindStr << (void *)Vtable;