[clang] [llvm] [mlir] [libc] [compiler-rt] [clang-tools-extra] [libcxx] [flang] Make SmallVectorImpl destructor protected (PR #71439)

[David Blaikie via cfe-commits]

dwblaikie wrote:

We probably (pretty sure) don't want to add a virtual dtor to SmallVector - that'd add a vtable pointer, increasing the size in ways that are probably unacceptable given the pervasive use of the data structure.

We should make the Impl dtor protected so it can't be polymorphically destroyed.

https://github.com/llvm/llvm-project/pull/71439