tsan: fix latent bug in shadow computation

[dvyukov]

Summary

We use kShadowCnt (number of shadow cells per application granule)
when computing shadow, but it's wrong. We need the ratio
between shadow and app memory (how much shadow is larger than app memory),
which is kShadowMultiplier.
Currently both are equal to 4, so it works fine.
Use the correct constant.

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

 }
 
 struct MemToShadowImpl {
   template <typename Mapping>
   static uptr Apply(uptr x) {
     DCHECK(IsAppMemImpl::Apply<Mapping>(x));
     return (((x) & ~(Mapping::kShadowMsk | (kShadowCell - 1))) ^
             Mapping::kShadowXor) *
-               kShadowCnt +
+               kShadowMultiplier +
            Mapping::kShadowAdd;
   }
 };
 
 ALWAYS_INLINE
 RawShadow *MemToShadow(uptr x) {
   return reinterpret_cast<RawShadow *>(SelectMapping<MemToShadowImpl>(x));
 }
struct ShadowToMemImpl {
   static uptr Apply(uptr sp) {
     if (!IsShadowMemImpl::Apply<Mapping>(sp))
       return 0;
     // The shadow mapping is non-linear and we've lost some bits, so we don't
     // have an easy way to restore the original app address. But the mapping is
     // a bijection, so we try to restore the address as belonging to
     // low/mid/high range consecutively and see if shadow->app->shadow mapping
     // gives us the same address.
-    uptr p = ((sp - Mapping::kShadowAdd) / kShadowCnt) ^ Mapping::kShadowXor;
+    uptr p =
+        ((sp - Mapping::kShadowAdd) / kShadowMultiplier) ^ Mapping::kShadowXor;
     if (p >= Mapping::kLoAppMemBeg && p < Mapping::kLoAppMemEnd &&
         MemToShadowImpl::Apply<Mapping>(p) == sp)
       return p;
     if (Mapping::kMidAppMemBeg) {
       uptr p_mid = p + (Mapping::kMidAppMemBeg & Mapping::kShadowMsk);
       if (p_mid >= Mapping::kMidAppMemBeg && p_mid < Mapping::kMidAppMemEnd &&
           MemToShadowImpl::Apply<Mapping>(p_mid) == sp)
         return p_mid;