c++: Fix spurious fallthrough warning on break

c++: Fix spurious fallthrough warning on break

The C++ frontend generates a break that results in the fallthrough
warning misfiring in nested switch blocks where cases in the inner
switch block return, rendering the break pointless.  The fallthrough
detection in finish_break_stmt does not work either because the
condition is encoded as an IF_STMT and not a COND_EXPR.

Fix this by adding a condition for IF_STMT in the
langhooks.block_may_fallthru for C++.  Fix tested on x86_64.

gcc/cp
        * cp-objcp-common.c (cxx_block_may_fallthru): Add case for
	        IF_STMT.

gcc/testsuite
        * g++.dg/nested-switch.C: New test case.

From-SVN: r257843