c/c++: use positive tone in missing header notes [PR84890]

c/c++: use positive tone in missing header notes [PR84890]

Quoting "How a computer should talk to people" (as quoted
in "Concepts Error Messages for Humans"):

"Various negative tones or actions are unfriendly: being manipulative,
not giving a second chance, talking down, using fashionable slang,
blaming. We must not seem to blame the person. We should avoid suggesting
that the person is inadequate. Phrases like "you forgot" may seem
harmless, but what if a computer said this to you four or five times in
two minutes? Anyway, the person may disagree, so why risk offense?"

gcc/c-family/ChangeLog:
	PR c/84890
	* known-headers.cc
	(suggest_missing_header::~suggest_missing_header): Reword note to
	avoid negative tone of "forgetting".

gcc/cp/ChangeLog:
	PR c/84890
	* name-lookup.cc (missing_std_header::~missing_std_header): Reword
	note to avoid negative tone of "forgetting".

gcc/testsuite/ChangeLog:
	PR c/84890
	* g++.dg/cpp2a/srcloc3.C: Update expected message.
	* g++.dg/lookup/missing-std-include-2.C: Likewise.
	* g++.dg/lookup/missing-std-include-3.C: Likewise.
	* g++.dg/lookup/missing-std-include-6.C: Likewise.
	* g++.dg/lookup/missing-std-include.C: Likewise.
	* g++.dg/spellcheck-inttypes.C: Likewise.
	* g++.dg/spellcheck-stdint.C: Likewise.
	* g++.dg/spellcheck-stdlib.C: Likewise.
	* gcc.dg/spellcheck-inttypes.c: Likewise.
	* gcc.dg/spellcheck-stdbool.c: Likewise.
	* gcc.dg/spellcheck-stdint.c: Likewise.
	* gcc.dg/spellcheck-stdlib.c: Likewise.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>