Add missing ATTRIBUTE_NORETURNs

This patch adds a missing ATTRIBUTE_NORETURN.  This lets
-Wimplicit-fallthrough recognize that a given case does not fall
through.

ChangeLog
2018-05-04  Tom Tromey  <tom@tromey.com>

	* dwarf2loc.c (unimplemented): Add ATTRIBUTE_NORETURN.

gdb/ChangeLog

@@ -1,3 +1,7 @@
+2018-05-04  Tom Tromey  <tom@tromey.com>
+
+	* dwarf2loc.c (unimplemented): Add ATTRIBUTE_NORETURN.
+
 2018-05-04  Tom Tromey  <tom@tromey.com>
 
 	* s390-tdep.c (s390_process_record): Fix fall-through comments.

gdb/dwarf2loc.c

@@ -2865,7 +2865,7 @@ dwarf2_loc_desc_get_symbol_read_needs (const gdb_byte *data, size_t size,
 /* A helper function that throws an unimplemented error mentioning a
    given DWARF operator.  */
 
-static void
+static void ATTRIBUTE_NORETURN
 unimplemented (unsigned int op)
 {
   const char *name = get_DW_OP_name (op);