Fix problems with the MSP430 port's handling of persistent data.

	PR target/78818
gcc	* config/msp430/msp430.c (msp430_data_attr): Check that it's possible
	for a variable to have a section before checking if the section has a
	name.
	Set section to.persistent if persistent	attribute is set.
	Warn if .persistent attribute is used on an automatic variable.

tests	* gcc.target/msp430/pr78818-real.c: New template for tests.
	* gcc.target/msp430/pr78818-auto.c: New test.
	* gcc.target/msp430/pr78818-data-region.c: New test.
	* gcc.target/msp430/pr78818-data-sec.c: New test.
	* gcc.target/msp430/pr78818-auto-warn.c: New test.

From-SVN: r249222