[EH] Tidy up format_exception.cpp (NFC)

[aheejin]

This makes small cosmetic changes that do not affect the functionality.

• Use using namespace __cxxabiv1. This namespace is used many times
  and I think it's OK to put this at the top. Other libc++abi cpp files
  also do this.
• Use static_cast instead of the old cast. This is also in line with
  the rest of libc++abi.
• Change some variable names. This tries to make variable names in line
  with libc++abi's other files; personally I don't particularly like
  their long names such as exception_header, but I think keeping them
  in line with the rest of the code is generally good and possibly
  helpful when we upstream the changes.
• Add cxa_exception_from_thrown_object and use it. The reason I copied
  this function from cxa_exception.cpp is I'm planning to use all these
  three when I add Wasm EH support for this function, and it will be
  easier to read if we use these than just doing the arthmetic
  ourselves.
  

  emscripten/system/lib/libcxxabi/src/cxa_exception.cpp

  Lines 39 to 67 in d5ef693

  	// Utility routines
  	static
  	inline
  	__cxa_exception*
  	cxa_exception_from_thrown_object(void* thrown_object)
  	{
  	return static_cast<__cxa_exception*>(thrown_object) - 1;
  	}
  	// Note: This is never called when exception_header is masquerading as a
  	// __cxa_dependent_exception.
  	static
  	inline
  	void*
  	thrown_object_from_cxa_exception(__cxa_exception* exception_header)
  	{
  	return static_cast<void*>(exception_header + 1);
  	}
  	// Get the exception object from the unwind pointer.
  	// Relies on the structure layout, where the unwind pointer is right in
  	// front of the user's exception object
  	static
  	inline
  	__cxa_exception*
  	cxa_exception_from_exception_unwind_exception(_Unwind_Exception* unwind_exception)
  	{
  	return cxa_exception_from_thrown_object(unwind_exception + 1 );
  	}

  
  The reason I copied them is they are static functions in
  cxa_exception.cpp. We possibly can move them to a header file and
  include it both places, but that means further changes to libc++abi
  codebase, which I think will make this harder to upstream.
• Use __shim_type_info::can_catch directly instead of __cxa_catch,
  which is a wrapper created to be called from JS. This removes the need
  to include the declaration of __cxa_catch in this file.

[aheejin]

cc @hoodmane

[sbc100]

I have another change that also adds this function: #16627

But I don't mind merging them when I land that.