[test][libc] Fix aligned_alloc argument

[vitalybuka]

Summary

Size must be multiple of Alignment.

[vitalybuka]

use alignTo

[gchatelet]

Thx for catching this !

libc/benchmarks/LibcMemoryBenchmark.h

 #ifndef LLVM_LIBC_UTILS_BENCHMARK_MEMORY_BENCHMARK_H
 #define LLVM_LIBC_UTILS_BENCHMARK_MEMORY_BENCHMARK_H
 
 #include "LibcBenchmark.h"
 #include "LibcFunctionPrototypes.h"
 #include "MemorySizeDistributions.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/Support/Alignment.h"
+#include "llvm/Support/MathExtras.h"
 #include <cstdint>
 #include <optional>
 #include <random>
 
 namespace llvm {
 namespace libc_benchmarks {
 
 //--------------

 //------
 
 // Provides an aligned, dynamically allocated buffer.
 class AlignedBuffer {
   char *const Buffer = nullptr;
   size_t Size = 0;
 
 public:
-  // Note: msan / asan can't handle Alignment > 512.
   static constexpr size_t Alignment = 512;
 
   explicit AlignedBuffer(size_t Size)
-      : Buffer(static_cast<char *>(aligned_alloc(Alignment, Size))),
+      : Buffer(static_cast<char *>(
+            aligned_alloc(Alignment, alignTo(Size, Alignment)))),
         Size(Size) {}
   ~AlignedBuffer() { free(Buffer); }
 
   inline char *operator+(size_t Index) { return Buffer + Index; }
   inline const char *operator+(size_t Index) const { return Buffer + Index; }
   inline char &operator[](size_t Index) { return Buffer[Index]; }
   inline const char &operator[](size_t Index) const { return Buffer[Index]; }
   inline char *begin() { return Buffer; }