[WebAssembly] Stop generating helper functions in WebAssemblyLowerEmscriptenEHSjLj

Previously we were creating weakly defined helper function in
each translation unit:

-  setThrew
-  setTempRet0

Instead we now assume these will be provided at link time.  In
emscripten they are provided in compiler-rt:
 https://github.com/kripken/emscripten/pull/7203

Additionally we previously created three global variable which are
also now required to exist at link time instead.

- __THREW__
- _threwValue
- __tempRet0

Differential Revision: https://reviews.llvm.org/D49208

llvm-svn: 343640