Implement Levenshtein distance; use in C FE for misspelled field names

Implement Levenshtein distance; use in C FE for misspelled field names

This is the combination of:
  [PATCH 1/2] Implement Levenshtein distance
  [PATCH 2/2] C FE: suggest corrections for misspelled field names
plus some nit fixes in spellcheck.c.

gcc/ChangeLog:
	* Makefile.in (OBJS): Add spellcheck.o.
	* spellcheck.c: New file.
	* spellcheck.h: New file.

gcc/c/ChangeLog:
	* c-typeck.c: Include spellcheck.h.
	(lookup_field_fuzzy_find_candidates): New function.
	(lookup_field_fuzzy): New function.
	(build_component_ref): If the field was not found, try using
	lookup_field_fuzzy and potentially offer a suggestion.

gcc/testsuite/ChangeLog:
	* gcc.dg/plugin/levenshtein-test-1.c: New file.
	* gcc.dg/plugin/levenshtein_plugin.c: New file.
	* gcc.dg/plugin/plugin.exp (plugin_test_list): Add
	levenshtein_plugin.c.
	* gcc.dg/spellcheck-fields.c: New file.

From-SVN: r230284