lookup_name_fuzzy can offer some reserved words as suggestions for
misspelled words, helping with "singed"/"signed" typos.

PR c++/81610 and PR c++/80567 report problems where the C++ frontend
suggested "if", "for" and "else" as corrections for misspelled variable
names.

The root cause is that in r247233
  ("Fix spelling suggestions for reserved words (PR c++/80177)")
I loosened the conditions on these reserved words, adding this condition:
   if (kind == FUZZY_LOOKUP_TYPENAME)
to the logic for rejecting words that don't start decl-specifiers, to
allow for "static_assert" to be offered.

This is too loose a condition: we don't want to suggest *any* reserved word
when we're in a context where we don't know we expect a typename.

For the kinds of error-recover situations where we're suggesting
spelling corrections we don't have much contextual information, so it
seems prudent to be stricter about which reserved words we offer
as spelling suggestions; I don't think it makes sense for us to
suggest e.g. "for".

This patch implements that by effectively reinstating the old logic,
but special-casing RID_STATIC_ASSERT, moving the logic to a new
subroutine (in case we want to allow for other special-cases).

I attempted to add suggestions for the various RID_*CAST, to cope
with e.g. "reinterptet_cast" (I can never type that correctly on the
first try), but the following '<' token confuses the error-recovery
enough that the suggestion code isn't triggered.

gcc/cp/ChangeLog:
	PR c++/81610
	PR c++/80567
	* name-lookup.c (suggest_rid_p): New function.
	(lookup_name_fuzzy): Replace enum-rid-filtering logic with call to
	suggest_rid_p.

gcc/testsuite/ChangeLog:
	PR c++/81610
	PR c++/80567
	* g++.dg/spellcheck-reswords.C: New test case.
	* g++.dg/spellcheck-stdlib.C: Remove xfail from dg-bogus
	suggestion of "if".

From-SVN: r257456