diagnostics: show an extra line of context in line-insertion fix-it hints (PR 87091)

diagnostics: show an extra line of context in line-insertion fix-it hints (PR 87091)

This patch tweaks how we print line-insertion fix-it hints, so that
the line before the insertion point is also printed, to give the user
more context on the proposed change.

For example, it changes:

  ../x86_64-pc-linux-gnu/libstdc++-v3/include/vector:87:22: note: message
  +++ |+#include <vector>
   74 | #endif
  ....
   87 |       using vector = std::vector<_Tp, polymorphic_allocator<_Tp>>;
      |                      ^~~

to:

  ../x86_64-pc-linux-gnu/libstdc++-v3/include/vector:87:22: note: message
   73 | # include <debug/vector>
  +++ |+#include <vector>
   74 | #endif
  ....
   87 |       using vector = std::vector<_Tp, polymorphic_allocator<_Tp>>;
      |                      ^~~

gcc/ChangeLog:
	PR 87091
	* diagnostic-show-locus.c (get_line_span_for_fixit_hint): Show the
	line above for line-insertion fix-it hints.
	(selftest::test_fixit_insert_containing_newline): Update the
	expected results, and add a test with line-numbering enabled.

gcc/testsuite/ChangeLog:
	PR 87091
	* g++.dg/pr85523.C: Extend expected output to show line
	before line-insertion fix-it hint.
	* gcc.dg/plugin/diagnostic-test-show-locus-bw-line-numbers.c
	(test_fixit_insert_newline): Add previous line to expected output.
	* gcc.dg/plugin/diagnostic-test-show-locus-bw.c: Likewise.
	* gcc.dg/plugin/diagnostic-test-show-locus-color.c: Likewise.

From-SVN: r263884