[clang] [llvm] [Clang][LLVM] Add -finline-stringops=memset and inline-stringops attribute (PR #231163)

[Arth Srivastava via cfe-commits]

Blazearth wrote:

>The LLVM IR attributes here look overly complicated. There's a very small list of operations which actually make sense to disable this way; we can just add a separate attribute for each such operation.

Hey @efriedma-quic thanks for the review! I've refactored the patch to use a dedicated `inline-memset` boolean attribute with the standard `setOR<InlineMemsetAttr>` merge rule, removing the custom attribute-merging logic. 
>All attributes need to be documented in LangRef.

I've also documented the attribute in LangRef and updated the relevant tests.

I kept it separate from `no-builtin-memset` because disabling builtin recognition and requiring inline expansion have different semantics, and reusing the existing attribute could change code generation for current users.

https://github.com/llvm/llvm-project/pull/231163