DWARF: for variants, produce unsigned discr. when debug type is unsigned

DWARF: for variants, produce unsigned discr. when debug type is unsigned

In Ada, the Character type is supposed to be unsigned.  However,
depending on the sign of C char types, GNAT can materialize it as a
signed type for code generation purposes.  When this is the case, GNAT
also attach a debug type to it so it is represented as an unsigned base
type in the debug information.

This change adapts record variant parts processing in the DWARF back-end
so that when the debug type of discriminant is unsigned while
discriminant values are signed themselves, we output unsigned
discriminant values in DWARF.

gcc/

	* dwarf2out.c (get_discr_value): Call the get_debug_type hook on
	the type of the input discriminant value.  Convert the
	discriminant value of signedness vary.

gcc/testsuite/

	* gnat.dg/debug11.adb: New testcase.

From-SVN: r248773