[llvm] [RFC][Support] Introduce buffered_svector_ostream (PR #97704)

[Alexis Engelke via llvm-commits]

aengelke wrote:

> Is there an estimate of the in-tree violations?

This PR includes a sample -- I would estimate that maybe half of the cases (mostly code written past 2015) need code changes (calling destructor/syncing; use tell instead of size). There are a few cases where this is not completely obvious transformation (read: needs context across multiple functions/files).

> If the conceived out-of-tree breakage is small, we could make this part of LLVM_ENABLE_ABI_BREAKING_CHECKS initially, and possibly call `__asan_poison_memory_region` to detect `[tell(), size())` access (due to accessing `SmallVector` before synchronization).

That could work, yes. write() would become very expensive in sanitizer builds, though.

https://github.com/llvm/llvm-project/pull/97704