C: don't suggest names that came from earlier failures (PR c/83056)

C: don't suggest names that came from earlier failures (PR c/83056)

PR c/83056 reports an issue affecting trunk and gcc-7 in which
the C frontend's implementation of lookup_name_fuzzy uses undeclared
identifiers as suggestions when encountering subsequent undeclared
identifiers.

The fix is to filter out the names bound to error_mark_node
in lookup_name_fuzzy.

The C++ frontend is unaffected, as it already does this.

gcc/c/ChangeLog:
	PR c/83056
	* c-decl.c (lookup_name_fuzzy): Don't suggest names that came from
	earlier failed lookups.

gcc/testsuite/ChangeLog:
	PR c/83056
	* gcc.dg/spellcheck-pr83056.c: New test case.

From-SVN: r255038