[llvm] [WebAssembly] Allocate MCSymbolWasm data on MCContext. (PR #85866)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Fixes https://github.com/llvm/llvm-project/issues/85578. The added test passes locally with an LLVM build with AddressSanitizer enabled.
>
> Implementation notes:

Consider summarizing the issue in the description. There is a use-after-free as the signature backed by WebAssemblyAsmParser has been destroyed.

https://github.com/llvm/llvm-project/pull/85866