[WebAssembly] Make EH/SjLj vars unconditionally thread local

This makes three thread local variables (`__THREW__`, `__threwValue`,
and `__wasm_lpad_context`) unconditionally thread local. If the target
doesn't support TLS, they will be downgraded to normal variables in
`stripThreadLocals`. This makes the object not linkable with other
objects using shared memory, which is what we intend here; these
variables should be thread local when used with shared memory. This is
what we initially tried in D88262.

But D88323 changed this: It only created these variables when threads
were supported, because `__THREW__` and `__threwValue` were always
generated even if Emscripten EH/SjLj was not used, making all objects
built without threads not linkable with shared memory, which was too
restrictive. But sometimes this is not safe. If we build an object using
variables such as `__THREW__` without threads, it can be linked to other
objects using shared memory, because the original object's `__THREW__`
was not created thread local to begin with.

So this CL basically reverts D88323 with some additional improvements:
- This checks each of the functions and global variables created within
  `LowerEmscriptenEHSjLj` pass and removes it if it's not used at the
  end of the pass. So only modules using those variables will be
  affected.
- Moves `CoalesceFeaturesAndStripAtomics` and `AtomicExpand` passes
  after all other IR pasess that can create thread local variables. It
  is not sufficient to move them to the end of `addIRPasses`, because
  `__wasm_lpad_context` is created in `WasmEHPrepare`, which runs inside
  `addPassesToHandleExceptions`, which runs before `addISelPrepare`. So
  we override `addISelPrepare` and move atomic/TLS stripping and
  expanding passes there.

This also removes merges `TLS` and `NO-TLS` FileCheck lines into one
`CHECK` line, because in the bitcode level we always create them as
thread local. Also some function declarations are deleted `CHECK` lines
because they are unused.

Reviewed By: tlively, sbc100

Differential Revision: https://reviews.llvm.org/D120013