Fix locations within raw strings

Fix locations within raw strings

Whilst investigating PR preprocessor/78324 I noticed that the
substring location code currently doesn't handle raw strings
correctly, by not skipping the 'R', opening quote, delimiter
and opening parenthesis.

For example, an attempt to underline chars 4-7 with caret at 6 of
this raw string yields this erroneous output:
   __emit_string_literal_range (R"foo(0123456789)foo",
                                    ~~^~

With the patch, the correct range/caret is printed:

   __emit_string_literal_range (R"foo(0123456789)foo",
                                          ~~^~

gcc/ChangeLog:
	* input.c (selftest::test_lexer_string_locations_long_line): New
	function.
	(selftest::test_lexer_string_locations_raw_string_multiline): New
	function.
	(selftest::input_c_tests): Call the new functions, via
	for_each_line_table_case.

gcc/testsuite/ChangeLog:
	* gcc.dg/plugin/diagnostic-test-string-literals-1.c
	(test_raw_string_one_liner): New function.
	(test_raw_string_multiline): New function.

libcpp/ChangeLog:
	* charset.c (cpp_interpret_string_1): Skip locations from
	loc_reader when advancing 'p' when handling raw strings.

From-SVN: r242552