[llvm] [memprof] Use std::vector<Frame> instead of llvm::SmallVector<Frame> (NFC) (PR #94432)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Consider `SmallVector<Frame, 0>`? `sizeof(SmallVector<X, 0>) < sizeof(vector<X>)`

https://github.com/llvm/llvm-project/pull/94432