mem,gpu-compute: Stop using the GEM5_NO_DISCARD macro.

The [[nodiscard]] attribute is now standard, so we can use that
directly.

Change-Id: I57f59935858facb2a15bf4712be4bfd584bf0c7e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48509
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/gpu-compute/compute_unit.hh

@@ -479,7 +479,7 @@ class ComputeUnit : public ClockedObject
     int32_t
     getRefCounter(const uint32_t dispatchId, const uint32_t wgId) const;
 
-    GEM5_NO_DISCARD bool sendToLds(GPUDynInstPtr gpuDynInst);
+    [[nodiscard]] bool sendToLds(GPUDynInstPtr gpuDynInst);
 
     typedef std::unordered_map<Addr, std::pair<int, int>> pageDataStruct;
     pageDataStruct pageAccesses;

src/mem/cache/base.hh

@@ -808,7 +808,7 @@ class BaseCache : public ClockedObject
      * @param blk Block to invalidate
      * @return A packet with the writeback, can be nullptr
      */
-    GEM5_NO_DISCARD virtual PacketPtr evictBlock(CacheBlk *blk) = 0;
+    [[nodiscard]] virtual PacketPtr evictBlock(CacheBlk *blk) = 0;
 
     /**
      * Evict a cache block.

src/mem/cache/cache.hh

@@ -137,7 +137,7 @@ class Cache : public BaseCache
     uint32_t handleSnoop(PacketPtr pkt, CacheBlk *blk,
                          bool is_timing, bool is_deferred, bool pending_inval);
 
-    GEM5_NO_DISCARD PacketPtr evictBlock(CacheBlk *blk) override;
+    [[nodiscard]] PacketPtr evictBlock(CacheBlk *blk) override;
 
     /**
      * Create a CleanEvict request for the given block.

src/mem/cache/noncoherent_cache.hh

@@ -120,7 +120,7 @@ class NoncoherentCache : public BaseCache
                                bool needs_writable,
                                bool is_whole_line_write) const override;
 
-    GEM5_NO_DISCARD PacketPtr evictBlock(CacheBlk *blk) override;
+    [[nodiscard]] PacketPtr evictBlock(CacheBlk *blk) override;
 
   public:
     NoncoherentCache(const NoncoherentCacheParams &p);