c++: Redundant -Wdeprecated-declarations warning in build_over_call [PR67960]

c++: Redundant -Wdeprecated-declarations warning in build_over_call [PR67960]

In build_over_call, we are emitting a redundant -Wdeprecated-declarations
warning about the deprecated callee function, first from mark_used and again
from build_addr_func <- decay_conversion <- cp_build_addr_expr <- mark_used.

It seems this second deprecation warning coming from build_addr_func will always
be redundant, so we can safely use a warning_sentinel to disable it before
calling build_addr_func.  (And any deprecation warning that could come from
build_addr_func would be for FN, so we wouldn't be suppressing too much.)

gcc/cp/ChangeLog:

	PR c++/67960
	* call.c (build_over_call): Use a warning_sentinel to disable
	warn_deprecated_decl before calling build_addr_func.

gcc/testsuite/ChangeLog:

	PR c++/67960
	* g++.dg/diagnostic/pr67960.C: New test.
	* g++.dg/diagnostic/pr67960-2.C: New test.