PR c/71171: Fix uninitialized source_range in c_parser_postfix_expression

PR c/71171: Fix uninitialized source_range in c_parser_postfix_expression

A common way for a c_expr to have an uninitialized src_range is in
error-handling, where the "value" field is set to error_mark_node
without touching the src_range, leading to complaints from valgrind.

This patch introduces a new method c_expr::set_error which sets
the value to error_mark_node whilst initializing the src_range to
UNKNOWN_LOCATION.

This fixes the valgrind issue seen in PR c/71171, along with various
other related issues seen when running the testsuite using the
checker patch I posted here:
  https://gcc.gnu.org/ml/gcc-patches/2015-12/msg00887.html
(this checker still doesn't fully work yet, but it seems to be good
for easily detecting these issues without needing Valgrind).

gcc/c/ChangeLog:
	PR c/71171
	* c-parser.c (c_parser_generic_selection): Use c_expr::set_error
	in error-handling.
	(c_parser_postfix_expression): Likewise.
	* c-tree.h (c_expr::set_error): New method.
	* c-typeck.c (parser_build_binary_op): In error-handling, ensure
	that result's range is initialized.

From-SVN: r236488