mangle.c (write_discriminator): Use `_0' for discriminator 1, not `_'.

	* mangle.c (write_discriminator): Use `_0' for discriminator 1,
	not `_'.

	* cp-demangle.c (demangle_discriminator): `_0' is discriminator #1,
	`_' not followed by a digit is invalid.

From-SVN: r40795