[MC] .addrsig_sym: ignore unregistered symbols

.addrsig_sym forces registering the symbol regardless whether it is otherwise
registered. This creates an undefined symbol which is inconvenient/undesired:

* `extern int x; void f() { (void)x; }` has inconsistent behavior whether `x` is emitted as an undefined symbol.
  `-O0 -faddrsig` makes `x` undefined while other -O levels and -fno-addrsig eliminate the symbol.
* In ThinLTO, after a non-prevailing linkonce_odr definition is converted to available_externally, and then a declaration,
  the addrsig code emits a symbol while the symbol is otherwise unseen.

D135427 fixed a bug that a non-prevailing `__cxx_global_var_init` was
incorrectly retained. However, the IR declaration causes an undesired
`.addrsig_sym __cxx_global_var_init`. This can be addressed in a way similar
to D101512 (`isTransitiveUsedByMetadataOnly`) but the increased
`OutStreamer->emitAddrsigSym(getSymbol(&GV));` complexity makes me nervous.
Just ignoring unregistered symbols circumvents the problem.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D135642