msan: update shadow constant in strlen_of_shadow test

[thurston]

Summary

strlen_of_shadow hardcodes the mem-to-shadow
constant. That constant was changed in D137666;
this CL updates the test accordingly.
(Other tests will be updated separately.)

[thurston]

@vitalybuka I pushed this simple change without review, to (hopefully) fix the build. Please provide post-commit review.

[DavidSpickett]

I don't think we got around to reporting it, but anyway, this fixed our AArch64 SVE bot - https://lab.llvm.org/buildbot/#/builders/197/builds/3475. Thanks!

compiler-rt/test/msan/strlen_of_shadow.cpp

#elif defined (__mips64)
   return (char *)((uintptr_t)p ^ 0x8000000000ULL);
 #elif defined(__powerpc64__)
 #define LINEARIZE_MEM(mem) \
   (((uintptr_t)(mem) & ~0x200000000000ULL) ^ 0x100000000000ULL)
   return (char *)(LINEARIZE_MEM(p) + 0x080000000000ULL);
 #elif defined(__s390x__)
   return (char *)(((uintptr_t)p & ~0xC00000000000ULL) + 0x080000000000ULL);
 #elif defined(__aarch64__)
-  return (char *)((uintptr_t)p ^ 0x6000000000ULL);
+  return (char *)((uintptr_t)p ^ 0xB00000000000ULL);
 #endif
 }
 
 int main(void) {
   const char *s = "abcdef";
   assert(strlen(s) == 6);
   assert(strlen(mem_to_shadow(s)) == 0);
 
   char *t = new char[42];
   t[41] = 0;
   assert(strlen(mem_to_shadow(t)) == 41);
   return 0;
 }