[WebAssembly] Use default attributes for intrinsics

This switches wasm intrinsics to use default attributes,
i.e. nofree, nosync, nocallback and willreturn. Especially
willreturn will be required to avoid optimization regressions
in the future.

The attributes are omitted from the trapping fptoi intrinsics
(where I assume trapping is considered well-defined, and as such
these aren't willreturn), the throw/rethrow intrinsics (which
will unwind) and the atomic intrinsics (which aren't nosync).

Differential Revision: https://reviews.llvm.org/D137551

Author: nikic