analyzer: only look for named functions in root ns [PR107788]

gcc/analyzer/ChangeLog:
	PR analyzer/107788
	* known-function-manager.cc (known_function_manager::get_match):
	Don't look up fndecls by name when they're not in the root
	namespace.

gcc/testsuite/ChangeLog:
	PR analyzer/107788
	* g++.dg/analyzer/named-functions.C: New test.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>