Force user provided warning and error messages to only occupy one line.

Force user provided warning and error messages to only occupy one line.

 	PR 84195
gcc: 	* tree.c (escaped_string): New class.  Converts an unescaped
 	string into its escaped equivalent.
 	(warn_deprecated_use): Use the new class to convert the
 	deprecation message, if present.
 	(test_escaped_strings): New self test.
 	(test_c_tests): Add test_escaped_strings.
 	* doc/extend.texi (deprecated): Add a note that the
 	deprecation message is affected by the -fmessage-length
 	option, and that control characters will be escaped.
 	(#pragma GCC error): Document this pragma.
 	(#pragma GCC warning): Likewise.
 	* doc/invoke.texi (-fmessage-length): Document this option's
 	effect on the #warning and #error preprocessor directives and
 	the deprecated attribute.

testsuite;
 	* gcc.c-torture/compile/pr84195.c: New test.

From-SVN: r261633