[BOLT] Replace LLVM_ATTRIBUTE_UNUSED with [[maybe_unused]] (NFC) (#163700)

This patch replaces LLVM_ATTRIBUTE_UNUSED with [[maybe_unused]],
introduced as part of C++17.

Author: kazutakahirata

bolt/lib/Core/CallGraph.cpp

@@ -22,7 +22,7 @@
 #undef USE_SSECRC
 #endif
 
-static LLVM_ATTRIBUTE_UNUSED inline size_t hash_int64_fallback(int64_t k) {
+[[maybe_unused]] static inline size_t hash_int64_fallback(int64_t k) {
   uint64_t key = (unsigned long long)k;
   // "64 bit Mix Functions", from Thomas Wang's "Integer Hash Function."
   // http://www.concentric.net/~ttwang/tech/inthash.htm
@@ -35,7 +35,7 @@ static LLVM_ATTRIBUTE_UNUSED inline size_t hash_int64_fallback(int64_t k) {
   return static_cast<size_t>(static_cast<uint32_t>(key));
 }
 
-static LLVM_ATTRIBUTE_UNUSED inline size_t hash_int64(int64_t k) {
+[[maybe_unused]] static inline size_t hash_int64(int64_t k) {
 #if defined(USE_SSECRC) && defined(__SSE4_2__)
   size_t h = 0;
   __asm("crc32q %1, %0\n" : "+r"(h) : "rm"(k));

bolt/lib/Core/DebugData.cpp

@@ -101,7 +101,7 @@ std::optional<AttrInfo> findAttributeInfo(const DWARFDie DIE,
   return findAttributeInfo(DIE, AbbrevDecl, *Index);
 }
 
-LLVM_ATTRIBUTE_UNUSED
+[[maybe_unused]]
 static void printLE64(const std::string &S) {
   for (uint32_t I = 0, Size = S.size(); I < Size; ++I) {
     errs() << Twine::utohexstr(S[I]);

bolt/lib/Rewrite/DWARFRewriter.cpp

@@ -69,7 +69,7 @@ static void printDie(const DWARFDie &DIE) {
 }
 
 /// Lazily parse DWARF DIE and print it out.
-LLVM_ATTRIBUTE_UNUSED
+[[maybe_unused]]
 static void printDie(DWARFUnit &DU, uint64_t DIEOffset) {
   uint64_t OriginalOffsets = DIEOffset;
   uint64_t NextCUOffset = DU.getNextUnitOffset();