c++: #pragma doesn't disable -Wunused-label [PR113582]

c++: #pragma doesn't disable -Wunused-label [PR113582]

The PR complains that

  void do_something(){
    #pragma GCC diagnostic push
    #pragma GCC diagnostic ignored "-Wunused-label"
    start:;
    #pragma GCC diagnostic pop
  } #1

doesn't work.  That's because we warn_for_unused_label only while we're
in finish_function, meaning we're at #1 where we're outside the #pragma
region.  We can use suppress_warning + warning_suppressed_p to fix this.

Note that I'm not using TREE_USED.  Propagating it in tsubst_stmt/LABEL_EXPR
from decl to label would mean that we don't warn in do_something2, but
I think we want the warning there: we're in a template and the goto is
a discarded statement.

	PR c++/113582

gcc/c-family/ChangeLog:

	* c-warn.cc (warn_for_unused_label): Don't warn if -Wunused-label has
	been suppressed for the label.

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_label_for_labeled_statement): suppress_warning
	if it's not enabled at input_location.
	* pt.cc (tsubst_stmt): Call copy_warning.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wunused-label-4.C: New test.