[clang] [Serialization] Use specialized decl hash function for GlobalDeclID (PR #95730)

[Anton Korobeynikov via cfe-commits]

================
@@ -230,7 +230,11 @@ template <> struct DenseMapInfo<clang::GlobalDeclID> {
   }
 
   static unsigned getHashValue(const GlobalDeclID &Key) {
-    return DenseMapInfo<DeclID>::getHashValue(Key.get());
+    // Our default hash algorithm for 64 bits integer may not be very good.
+    // In GlobalDeclID's case, it is pretty common that the lower 32 bits can
+    // be same.
+    return DenseMapInfo<uint32_t>::getHashValue(Key.getModuleFileIndex()) ^
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asl wrote:

Can you use `hash_combine` from `llvm/ADT/Hash.h`?

https://github.com/llvm/llvm-project/pull/95730
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