jit: fix string escaping

jit: fix string escaping

This patch fixes a bug in recording::string::make_debug_string in which
'\t' and '\n' were "escaped" by simply prepending a '\', thus emitting
'\' then '\n', rather than '\' then 'n'.  It also removes a hack that
determined if a string is to be escaped by checking for a leading '"',
by instead adding a flag.

gcc/jit/ChangeLog:
	* jit-recording.c (recording::context::new_string): Add "escaped"
	param and use it when creating the new recording::string instance.
	(recording::string::string): Add "escaped" param and use it to
	initialize m_escaped.
	(recording::string::make_debug_string): Replace check that first
	char is double-quote with use of m_escaped.  Fix escaping of
	'\t' and '\n'.  Set "escaped" on the result.
	* jit-recording.h (recording::context::new_string): Add "escaped"
	param.
	(recording::string::string): Add "escaped" param.
	(recording::string::m_escaped): New field.

gcc/testsuite/ChangeLog:
	* jit.dg/test-debug-strings.c (create_code): Add tests of
	string literal escaping.