[llvm] [ADT] Move shrink_and_clear to DenseMapBase (NFC) (PR #165103)

[Jakub Kuderski via llvm-commits]

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@@ -1168,6 +1146,20 @@ class SmallDenseMap
     deallocate_buffer(OldRep.Buckets, sizeof(BucketT) * OldRep.NumBuckets,
                       alignof(BucketT));
   }
+
+  std::pair<bool, unsigned> planShrinkAndClear() const {
+    unsigned NewNumBuckets = 0;
+    if (this->size()) {
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kuhar wrote:

can we either use `size() == 0` or `empty()`?

https://github.com/llvm/llvm-project/pull/165103