[WebAssembly] Fix LowerEmscriptenEHSjLj when there's only longjmp

Summary:
The pass incorrectly assumed if there's a longjmp declaration in the
module, there is also a setjmp function declaration. Fixed it, and now
the pass only converts longjmp and does not do any other transformation
when there's no setjmp declaration in the module.

Fixes PR39562.

Reviewers: jgravelle-google, sbc100

Subscribers: dschuff, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D54273

llvm-svn: 346445