[Support] Add SpecificBumpPtrAllocator::identifyObject (#100475)

This already exists in BumpPtrAllocatorImpl, but I would like to use it
from SpecificBumpPtrAllocator.

I also noticed there was no test for identifyObject so I added one.

llvm/include/llvm/Support/Allocator.h

@@ -435,6 +435,13 @@ public:
 
   /// Allocate space for an array of objects without constructing them.
   T *Allocate(size_t num = 1) { return Allocator.Allocate<T>(num); }
+
+  /// \return An index uniquely and reproducibly identifying
+  /// an input pointer \p Ptr in the given allocator.
+  /// Returns an empty optional if the pointer is not found in the allocator.
+  std::optional<int64_t> identifyObject(const void *Ptr) {
+    return Allocator.identifyObject(Ptr);
+  }
 };
 
 } // end namespace llvm

llvm/unittests/Support/AllocatorTest.cpp

@@ -208,6 +208,27 @@ TEST(AllocatorTest, TestSlowerSlabGrowthDelay) {
   EXPECT_EQ(SlabSize * GrowthDelay + SlabSize * 2, Alloc.getTotalMemory());
 }
 
+TEST(AllocatorTest, TestIdentifyObject) {
+  BumpPtrAllocator Alloc;
+
+  uint64_t *a = (uint64_t *)Alloc.Allocate(sizeof(uint64_t), alignof(uint64_t));
+  std::optional<int64_t> maybe_a_belongs = Alloc.identifyObject(a);
+  EXPECT_TRUE(maybe_a_belongs.has_value());
+  EXPECT_TRUE(*maybe_a_belongs >= 0);
+
+  uint64_t *b = nullptr;
+  std::optional<int64_t> maybe_b_belongs = Alloc.identifyObject(b);
+  EXPECT_FALSE(maybe_b_belongs);
+
+  // The default slab size is 4096 (or 512 uint64_t values). Custom slabs are
+  // allocated when the requested size is larger than the slab size.
+  uint64_t *c =
+      (uint64_t *)Alloc.Allocate(sizeof(uint64_t) * 1024, alignof(uint64_t));
+  std::optional<int64_t> maybe_c_belongs = Alloc.identifyObject(c);
+  EXPECT_TRUE(maybe_c_belongs.has_value());
+  EXPECT_TRUE(*maybe_c_belongs < 0);
+}
+
 // Mock slab allocator that returns slabs aligned on 4096 bytes.  There is no
 // easy portable way to do this, so this is kind of a hack.
 class MockSlabAllocator {