sarif output: add descriptions to fix-it hints (§3.55.2) [PR121986]

SARIF "fix" objects SHOULD have a "description" property (§3.55.2) that
describes the proposed fix, but currently GCC's SARIF output doesn't
support this, and we don't capture this anywhere internally as we build
fix-it hints in the compiler.

Currently we can have zero or more instances of fixit_hint associated
with a diagnostic, each representing an edit of a range of the source
code. Ideally we would have an internal API that allowed for associating
multiple fixes with a diagnostic, each with a description worded in terms
of the source language (e.g. "Fix 'colour' mispelling of field 'color'"),
and each consisting of multiple edited ranges.

For now, this patch extends the sarif output sink so that it
autogenerates descriptions of fix-it hints for simple cases of
insertion, deletion, and replacement of a single range
(e.g. "Replace 'colour' with 'color'").

gcc/ChangeLog:
	PR diagnostics/121986
	* diagnostics/sarif-sink.cc: Include "intl.h".
	(sarif_builder::make_message_describing_fix_it_hint): New.
	(sarif_builder::make_fix_object): Attempt to auto-generate a
	description for fix-it hints.

gcc/testsuite/ChangeLog:
	PR diagnostics/121986
	* gcc.dg/sarif-output/extra-semicolon.c: New test.
	* gcc.dg/sarif-output/extra-semicolon.py: New test.
	* gcc.dg/sarif-output/missing-semicolon.py: Verify the description
	of the insertion fix-it hint.
	* libgdiagnostics.dg/test-fix-it-hint-c.py: Verify the description
	of the replacement fix-it hint.

libcpp/ChangeLog:
	PR diagnostics/121986
	* include/rich-location.h (fixit_hint::deletion_p): New accessor.
	(fixit_hint::replacement_p): New accessor.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>

libcpp/include/rich-location.h

@@ -642,6 +642,8 @@ class fixit_hint
   size_t get_length () const { return m_len; }
 
   bool insertion_p () const { return m_start == m_next_loc; }
+  bool deletion_p () const { return m_len == 0; }
+  bool replacement_p () const { return m_len > 0 && m_start != m_next_loc; }
 
   bool ends_with_newline_p () const;