Enable -Wunused-local-typedefs when -Wall or -Wunused is on

Enable -Wunused-local-typedefs when -Wall or -Wunused is on

Now that the libstdc++ testsuite is run with -ftrack-macro-location by
default, this patch triggers the -Wunused-local-typedefs warning when
-Wunused (and -Wall) is turned on.

The patch has been reviewed and accepted[1] a while ago, but was waiting
for the -ftrack-macro-expansion work to go in first.

Bootstrapped and tested again on x86_64-unknown-linux-gnu against
trunk.

Applied to the mainline.

[1]: http://gcc.gnu.org/ml/gcc-patches/2011-09/msg00605.html

gcc/

	* opts.c (finish_options): Activate -Wunused-local-typedefs if
	-Wunused is activated.
	* doc/invoke.texi: Update blurb of -Wunused-local-typedefs.

From-SVN: r187167