Explicitly instantiate common `raw_hash_set` backing array functions.

Add explicit template instantiations for `AllocateBackingArray` and `DeallocateBackingArray` in `raw_hash_set.cc` for the `std::allocator<char>` and `alignof(size_t)` case. Add corresponding `extern template` declarations in `raw_hash_set.h`. This reduces compile times and linker input size by ensuring these common instantiations are only generated once.

PiperOrigin-RevId: 828501019
Change-Id: I2927e756f0cf8238d6455c6eee4473fff790435f

absl/container/internal/raw_hash_set.cc

diff --git a/absl/container/internal/raw_hash_set.cc b/absl/container/internal/raw_hash_set.cc
index 09076b4..ea3a3bf 100644
--- a/absl/container/internal/raw_hash_set.cc
+++ b/absl/container/internal/raw_hash_set.cc
@@ -2067,6 +2067,14 @@
 static_assert(MaxSooSlotSize() == 16);
 #endif
 
+template void* AllocateBackingArray<BackingArrayAlignment(alignof(size_t)),
+                                    std::allocator<char>>(void* alloc,
+                                                          size_t n);
+template void DeallocateBackingArray<BackingArrayAlignment(alignof(size_t)),
+                                     std::allocator<char>>(
+    void* alloc, size_t capacity, ctrl_t* ctrl, size_t slot_size,
+    size_t slot_align, bool had_infoz);
+
 }  // namespace container_internal
 ABSL_NAMESPACE_END
 }  // namespace absl

absl/container/internal/raw_hash_set.h

diff --git a/absl/container/internal/raw_hash_set.h b/absl/container/internal/raw_hash_set.h
index 147a756..faa7880 100644
--- a/absl/container/internal/raw_hash_set.h
+++ b/absl/container/internal/raw_hash_set.h
@@ -3703,6 +3703,14 @@
     bool);
 #endif
 
+extern template void* AllocateBackingArray<
+    BackingArrayAlignment(alignof(size_t)), std::allocator<char>>(void* alloc,
+                                                                  size_t n);
+extern template void DeallocateBackingArray<
+    BackingArrayAlignment(alignof(size_t)), std::allocator<char>>(
+    void* alloc, size_t capacity, ctrl_t* ctrl, size_t slot_size,
+    size_t slot_align, bool had_infoz);
+
 }  // namespace container_internal
 ABSL_NAMESPACE_END
 }  // namespace absl