This adds a writable attribute, which in conjunction with derferenceable(N) states that a spurious store of N bytes is introduced on function entry. This implies that this many bytes are writable without trapping or introducing data races. See https://llvm.org/docs/Atomics.html#optimization-outside-atomic for why the second point is important.
This attribute can be added to sret arguments. I believe Rust will also be able to use it for by-value (moved) arguments. Rust likely won't be able to use it for &mut arguments (tree borrows does not appear to allow spurious stores).
In this patch the new attribute is only used by LICM scalar promotion. However, the actual motivation for this is to fix a correctness issue in call slot optimization, which needs this attribute to avoid optimization regressions.
Followup to the discussion on D157499.