diagnostics: Fix add_misspelling_candidates [PR115440]

diagnostics: Fix add_misspelling_candidates [PR115440]

The option_map array for most entries contains just non-NULL opt0
    { "-Wno-", NULL, "-W", false, true },
    { "-fno-", NULL, "-f", false, true },
    { "-gno-", NULL, "-g", false, true },
    { "-mno-", NULL, "-m", false, true },
    { "--debug=", NULL, "-g", false, false },
    { "--machine-", NULL, "-m", true, false },
    { "--machine-no-", NULL, "-m", false, true },
    { "--machine=", NULL, "-m", false, false },
    { "--machine=no-", NULL, "-m", false, true },
    { "--machine", "", "-m", false, false },
    { "--machine", "no-", "-m", false, true },
    { "--optimize=", NULL, "-O", false, false },
    { "--std=", NULL, "-std=", false, false },
    { "--std", "", "-std=", false, false },
    { "--warn-", NULL, "-W", true, false },
    { "--warn-no-", NULL, "-W", false, true },
    { "--", NULL, "-f", true, false },
    { "--no-", NULL, "-f", false, true }
and so add_misspelling_candidates works correctly for it, but 3 out of
these,
    { "--machine", "", "-m", false, false },
    { "--machine", "no-", "-m", false, true },
and
    { "--std", "", "-std=", false, false },
use non-NULL opt1.  That says that
--machine foo
should map to
-mfoo
and
--machine no-foo
should map to
-mno-foo
and
--std c++17
should map to
-std=c++17
add_misspelling_canidates was not handling this, so it hapilly
registered say
--stdc++17
or
--machineavx512
(twice) as spelling alternatives, when those options aren't recognized.
Instead we support
--std c++17
or
--machine avx512
--machine no-avx512

The following patch fixes that.  On this particular testcase, we no longer
suggest anything, even when among the suggestion is say that
--std c++17
or
-std=c++17
etc.

2024-06-17  Jakub Jelinek  <jakub@redhat.com>

	PR driver/115440
	* opts-common.cc (add_misspelling_candidates): If opt1 is non-NULL,
	add a space and opt1 to the alternative suggestion text.

	* g++.dg/cpp1z/pr115440.C: New test.

(cherry picked from commit 96db57948b50f45235ae4af3b46db66cae7ea859)