Fix memory leak of pretty_printer prefixes

Fix memory leak of pretty_printer prefixes

We were rather sloppy about handling the ownership of prefixes for
pretty_printer, and this lead to a memory leak for any time a
diagnostic_show_locus call emits multiple line spans.

This showed up in "make selftest-valgrind" as:

3,976 bytes in 28 blocks are definitely lost in loss record 632 of 669
   at 0x4A0645D: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x1F08227: xmalloc (xmalloc.c:147)
   by 0x1F083E6: xvasprintf (xvasprintf.c:58)
   by 0x1E7EC7D: build_message_string(char const*, ...) (diagnostic.c:78)
   by 0x1E7F438: diagnostic_get_location_text(diagnostic_context*, expanded_location) (diagnostic.c:328)
   by 0x1E7FD54: default_diagnostic_start_span_fn(diagnostic_context*, expanded_location) (diagnostic.c:626)
   by 0x1EB3508: selftest::test_diagnostic_context::start_span_cb(diagnostic_context*, expanded_location) (selftest-diagnostic.c:57)
   by 0x1E89215: diagnostic_show_locus(diagnostic_context*, rich_location*, diagnostic_t) (diagnostic-show-locus.c:1992)
   by 0x1E8ECAD: selftest::test_fixit_insert_containing_newline_2(selftest::line_table_case const&) (diagnostic-show-locus.c:3044)
   by 0x1EB0606: selftest::for_each_line_table_case(void (*)(selftest::line_table_case const&)) (input.c:3525)
   by 0x1E8F3F5: selftest::diagnostic_show_locus_c_tests() (diagnostic-show-locus.c:3164)
   by 0x1E010BF: selftest::run_tests() (selftest-run-tests.c:88)

4,004 bytes in 28 blocks are definitely lost in loss record 633 of 669
   at 0x4A0645D: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x1F08227: xmalloc (xmalloc.c:147)
   by 0x1F083E6: xvasprintf (xvasprintf.c:58)
   by 0x1E7EC7D: build_message_string(char const*, ...) (diagnostic.c:78)
   by 0x1E7F438: diagnostic_get_location_text(diagnostic_context*, expanded_location) (diagnostic.c:328)
   by 0x1E7FD54: default_diagnostic_start_span_fn(diagnostic_context*, expanded_location) (diagnostic.c:626)
   by 0x1EB3508: selftest::test_diagnostic_context::start_span_cb(diagnostic_context*, expanded_location) (selftest-diagnostic.c:57)
   by 0x1E89215: diagnostic_show_locus(diagnostic_context*, rich_location*, diagnostic_t) (diagnostic-show-locus.c:1992)
   by 0x1E8B373: selftest::test_diagnostic_show_locus_fixit_lines(selftest::line_table_case const&) (diagnostic-show-locus.c:2500)
   by 0x1EB0606: selftest::for_each_line_table_case(void (*)(selftest::line_table_case const&)) (input.c:3525)
   by 0x1E8F3B9: selftest::diagnostic_show_locus_c_tests() (diagnostic-show-locus.c:3159)
   by 0x1E010BF: selftest::run_tests() (selftest-run-tests.c:88)

This patch fixes the leaks by ensuring that the pretty_printer "owns"
the prefix if it's non-NULL, freeing it in the dtor and in pp_set_prefix.

gcc/cp/ChangeLog:
	* error.c (cxx_print_error_function): Duplicate "file" before
	passing it to pp_set_prefix.
	(cp_print_error_function): Use pp_take_prefix when saving the
	existing prefix.

gcc/ChangeLog:
	* diagnostic-show-locus.c (diagnostic_show_locus): Use
	pp_take_prefix when saving the existing prefix.
	* diagnostic.c (diagnostic_append_note): Likewise.
	* langhooks.c (lhd_print_error_function): Likewise.
	* pretty-print.c (pp_set_prefix): Drop the "const" from "prefix"
	param's type.  Free the existing prefix.
	(pp_take_prefix): New function.
	(pretty_printer::pretty_printer): Drop the prefix parameter.
	Rename the length parameter to match the comment.
	(pretty_printer::~pretty_printer): Free the prefix.
	* pretty-print.h (pretty_printer::pretty_printer): Drop the prefix
	parameter.
	(struct pretty_printer): Drop the "const" from "prefix" field's
	type and clarify memory management.
	(pp_set_prefix): Drop the "const" from the 2nd param.
	(pp_take_prefix): New decl.

From-SVN: r263275