c++: Fix invalid -Wduplicated-cond warning [PR94265]

c++: Fix invalid -Wduplicated-cond warning [PR94265]

This fixes a false-positive warning from -Wduplicate-cond in the presence of an
if-statement with a non-empty init-statement.  Precisely determining whether a
non-empty init-statement has side effects seems tricky and error-prone, so this
patch takes the route of unconditionally invalidating the condition chain when
it encounters such an if-statement.

gcc/cp/ChangeLog:

	PR c++/94265
	* parser.c (cp_parser_selection_statement) <case RID_IF>: Invalidate the
	current condition chain when the if-statement has a non-empty
	init-statement.

gcc/testsuite/ChangeLog:

	PR c++/94265
	* g++.dg/warn/Wduplicated-cond1.C: New test.