c++: libcpp: Support raw strings with newlines in directives [PR55971]

It's not currently possible to use a C++11 raw string containing a newline as
part of the definition of a macro, or in any other preprocessing directive,
such as:

 #define X R"(two
lines)"

 #error R"(this error has
two lines)"

Add support for that by relaxing the conditions under which
_cpp_get_fresh_line() refuses to get a new line. For the case of lexing a raw
string, it's OK to do so as long as there is another line within the current
buffer. The code in cpp_get_fresh_line() was refactored into a new function
get_fresh_line_impl(), so that the new logic is applied only when processing a
raw string and not any other times.

libcpp/ChangeLog:

	PR preprocessor/55971
	* lex.cc (get_fresh_line_impl): New function refactoring the code
	from...
	(_cpp_get_fresh_line): ...here.
	(lex_raw_string): Use the new version of get_fresh_line_impl() to
	support raw strings containing new lines when processing a directive.

gcc/testsuite/ChangeLog:

	PR preprocessor/55971
	* c-c++-common/raw-string-directive-1.c: New test.
	* c-c++-common/raw-string-directive-2.c: New test.

gcc/c-family/ChangeLog:

	PR preprocessor/55971
	* c-ppoutput.cc (adjust_for_newlines): Update comment.