[llvm] Replace LLVM_ATTRIBUTE_UNUSED with [[maybe_unused]] (NFC) (#163702)

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

Author: kazutakahirata

llvm/lib/Support/Windows/Signals.inc

@@ -354,8 +354,8 @@ namespace llvm {
 /// Emulates hitting "retry" from an "abort, retry, ignore" CRT debug report
 /// dialog. "retry" raises an exception which ultimately triggers our stack
 /// dumper.
-static LLVM_ATTRIBUTE_UNUSED int
-AvoidMessageBoxHook(int ReportType, char *Message, int *Return) {
+[[maybe_unused]] static int AvoidMessageBoxHook(int ReportType, char *Message,
+                                                int *Return) {
   // Set *Return to the retry code for the return value of _CrtDbgReport:
   // http://msdn.microsoft.com/en-us/library/8hyw4sy7(v=vs.71).aspx
   // This may also trigger just-in-time debugging via DebugBreak().

llvm/lib/Target/Mips/MipsFastISel.cpp

@@ -264,9 +264,10 @@ class MipsFastISel final : public FastISel {
 
 } // end anonymous namespace
 
-static bool CC_Mips(unsigned ValNo, MVT ValVT, MVT LocVT,
-                    CCValAssign::LocInfo LocInfo, ISD::ArgFlagsTy ArgFlags,
-                    Type *OrigTy, CCState &State) LLVM_ATTRIBUTE_UNUSED;
+[[maybe_unused]] static bool CC_Mips(unsigned ValNo, MVT ValVT, MVT LocVT,
+                                     CCValAssign::LocInfo LocInfo,
+                                     ISD::ArgFlagsTy ArgFlags, Type *OrigTy,
+                                     CCState &State);
 
 static bool CC_MipsO32_FP32(unsigned ValNo, MVT ValVT, MVT LocVT,
                             CCValAssign::LocInfo LocInfo,

llvm/lib/Target/Mips/MipsISelLowering.cpp

@@ -3176,9 +3176,10 @@ static bool CC_MipsO32_FP64(unsigned ValNo, MVT ValVT, MVT LocVT,
                     F64Regs);
 }
 
-static bool CC_MipsO32(unsigned ValNo, MVT ValVT, MVT LocVT,
-                       CCValAssign::LocInfo LocInfo, ISD::ArgFlagsTy ArgFlags,
-                       Type *OrigTy, CCState &State) LLVM_ATTRIBUTE_UNUSED;
+[[maybe_unused]] static bool CC_MipsO32(unsigned ValNo, MVT ValVT, MVT LocVT,
+                                        CCValAssign::LocInfo LocInfo,
+                                        ISD::ArgFlagsTy ArgFlags, Type *OrigTy,
+                                        CCState &State);
 
 #include "MipsGenCallingConv.inc"