[WebAssembly] Restore __builtin_wasm_rethrow builtin

Summary:
Restore the `__builtin_wasm_rethrow` builtin deleted in D37931. On second
thought, it appears it can be used to implement `__cxa_rethrow`.

Reviewers: dschuff, sunfish

Reviewed By: dschuff

Subscribers: jfb, sbc100, jgravelle-google

Differential Revision: https://reviews.llvm.org/D37942

llvm-svn: 313430