[WebAssembly] Fix JS code mentions in LowerEmscriptenEHSjLj

- Removes the mention of fastcomp, which is deprecated.
- Some functions in Emscripten have moved from JS glue code to
  compiler-rt/emscripten_setjmp.c and
  compiler-rt/emscripten_exception_builtins.c. This fixes comments about
  that.

Reviewed By: sbc100

Differential Revision: https://reviews.llvm.org/D101812