ipa-visibility: remove assert in TLS optimization [PR107353]

When upgrading TLS access model based on optimized symbol visibility
status, we attempted to assert that recomputing the model would not
weaken it. It turns out that C, C++, and Fortran front-ends all can
(unintentionally) assign a stronger model than what can be derived
from the declaration.

Let's act conservatively instead of asserting, at least as long as
such pre-existing issues remain.

gcc/ChangeLog:

	PR other/107353
	* ipa-visibility.cc (function_and_variable_visibility):
	Conditionally upgrade TLS model instead of asserting.