scudo: Shrink secondary header and cache entry size by a word on Linux. NFCI.

[pcc]

Summary

Normally compilers will allocate space for struct fields even if the
field is an empty struct. Use the [[no_unique_address]] attribute to
suppress that behavior. This attribute that was introduced in C++20,
but compilers that do not support [[no_unique_address]] will ignore
it since it uses C++11 attribute syntax.

[eugenis]

LGTM

compiler-rt/lib/scudo/standalone/secondary.h

 namespace LargeBlock {
 
 struct Header {
   LargeBlock::Header *Prev;
   LargeBlock::Header *Next;
   uptr BlockEnd;
   uptr MapBase;
   uptr MapSize;
-  MapPlatformData Data;
+  [[no_unique_address]] MapPlatformData Data;
 };
 
 constexpr uptr getHeaderSize() {
   return roundUpTo(sizeof(Header), 1U << SCUDO_MIN_ALIGNMENT_LOG);
 }
 
 static Header *getHeader(uptr Ptr) {
   return reinterpret_cast<Header *>(Ptr - getHeaderSize());
void releaseOlderThan(u64 Time) {
     }
   }
 
   struct CachedBlock {
     uptr Block;
     uptr BlockEnd;
     uptr MapBase;
     uptr MapSize;
-    MapPlatformData Data;
+    [[no_unique_address]] MapPlatformData Data;
     u64 Time;
   };
 
   HybridMutex Mutex;
   CachedBlock Entries[EntriesArraySize];
   u32 EntriesCount;
   atomic_u32 MaxEntriesCount;
   atomic_uptr MaxEntrySize;