substring_loc::get_location currently fails for the final terminator
character in a STRING_CST from the C frontend, so that format_warning_va
falls back to using the location of the string as a whole.

This patch tweaks things [1] so that we use the final closing quote
as the location of the terminator character, as requested in
PR preprocessor/77672.

[1] specifically, cpp_interpret_string_1.

gcc/ChangeLog:
	PR preprocessor/77672
	* input.c (selftest::test_lexer_string_locations_simple): Update
	test to expect location information of the terminator character
	at the location of the final closing quote.
	(selftest::test_lexer_string_locations_hex): Likewise.
	(selftest::test_lexer_string_locations_oct): Likewise.
	(selftest::test_lexer_string_locations_letter_escape_1): Likewise.
	(selftest::test_lexer_string_locations_letter_escape_2): Likewise.
	(selftest::test_lexer_string_locations_ucn4): Likewise.
	(selftest::test_lexer_string_locations_ucn8): Likewise.
	(selftest::test_lexer_string_locations_u8): Likewise.
	(selftest::test_lexer_string_locations_utf8_source): Likewise.
	(selftest::test_lexer_string_locations_concatenation_1): Likewise.
	(selftest::test_lexer_string_locations_concatenation_2): Likewise.
	(selftest::test_lexer_string_locations_concatenation_3): Likewise.
	(selftest::test_lexer_string_locations_macro): Likewise.
	(selftest::test_lexer_string_locations_long_line): Likewise.

gcc/testsuite/ChangeLog:
	PR preprocessor/77672
	* gcc.dg/plugin/diagnostic-test-string-literals-1.c
	(test_terminator_location): New function.

libcpp/ChangeLog:
	PR preprocessor/77672
	* charset.c (cpp_interpret_string_1): Add a source_range for the
	NUL-terminator, using the location of the trailing quote of the
	final string.

From-SVN: r240434