Prefer simple case changes in spelling suggestions

Prefer simple case changes in spelling suggestions

I got this error message when editing gcc and recompiling:

../../gcc/gcc/ada/gcc-interface/decl.c:7714:39: error: ‘DWARF_GNAT_ENCODINGS_all’ was not declared in this scope; did you mean ‘DWARF_GNAT_ENCODINGS_GDB’?
 7714 |     = debug_info && gnat_encodings == DWARF_GNAT_ENCODINGS_all;
      |                                       ^~~~~~~~~~~~~~~~~~~~~~~~
      |                                       DWARF_GNAT_ENCODINGS_GDB

This suggestion could be improved -- what happened here is that I
failed to upper-case the word, and DWARF_GNAT_ENCODINGS_ALL was the
correct spelling.

This patch changes gcc's spell checker to prefer simple case changes
when possible.

I tested this using the self-tests.  A new self-test is also included.

gcc/ChangeLog:

	* spellcheck.c (CASE_COST): New define.
	(BASE_COST): New define.
	(get_edit_distance): Recognize case changes.
	(get_edit_distance_cutoff): Update.
	(test_edit_distances): Update.
	(get_old_cutoff): Update.
	(test_find_closest_string): Add case sensitivity test.