[llvm] [ADT] Use StorageT for temporary storage in DenseMapBase::grow (NFC) (PR #228802)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay approved this pull request.

This part can be moved a separate comment so that it doesn't become part of the commit message
```
In a release build of clang, this patch reduces the size of
non-relocatable DenseMapBase::grow instantiations by 40% on average, in
turn reducing the .text section by 0.11% (75.76 MB to 75.67 MB).

Below is the geometric mean of the perf stat -r10 deltas (after 1 warmup
run) across SLPVectorizer.ii, SemaOpenMP.ii, and X86ISelLowering.ii with
clang -O3 -g -fdebug-info-for-profiling -flto=thin:

Metric          Geomean Delta
-----------------------------
instructions:u         -0.02%
cpu-cycles:u           -0.17%
time elapsed           -0.10%
```

https://github.com/llvm/llvm-project/pull/228802