[WebAssembly] EmscriptenEHSjLj: Don't abort if __THREW__ is defined

This allows __THREW__ to be defined in the current module, although
it is still required to be a GlobalVariable.

In emscripten we want to be able to compile the source code that
defines this symbols.

Previously we were avoid this by not running this pass when building
that compiler-rt library, but I have change out to build it using the
normal compiler path:
https://github.com/emscripten-core/emscripten/pull/8391

Differential Revision: https://reviews.llvm.org/D60232

llvm-svn: 357665