Emit DW_AT_GNU_bias with -fgnat-encodings=gdb.  gdb implements this,
but not the encoded variant.

2019-09-19  Tom Tromey  <tromey@adacore.com>

gcc/ada/

	* gcc-interface/misc.c (gnat_get_type_bias): Return the bias
	when -fgnat-encodings=gdb.

gcc/testsuite/

	* gnat.dg/bias1.adb: New testcase.

From-SVN: r275958