SS_Annotations: Change default backend for C/C++ sources

Stable_Sloc now offers a backend that can remap regions defined
within a declaration for C & C++ sources. Use this backend by default
for these languages.

(cherry picked from commit 78e9c47)

Author: leocreuse

testsuite/tests/ext_annotations/gen/invalidation/show_expected.txt

@@ -3,4 +3,4 @@ main.adb:
 - STALE ANNOTATION; id: exempt_ada; kind: Exempt_Region; Justification: Useless code; diagnostic: Contents of <SubpBody ["Nested"] main.adb:2:4-7:15> changed
 
 main_c.c:
-- STALE ANNOTATION; id: exempt_c; kind: Exempt_Region; Justification: Useless code; diagnostic: file has been modified
+- STALE ANNOTATION; id: exempt_c; kind: Exempt_Region; Justification: Useless code; diagnostic: Content of declaration ident has changed

tools/gnatcov/ss_annotations.adb

@@ -940,10 +940,13 @@ package body SS_Annotations is
          case Language is
             when Ada_Language => SS_Backend := +"lal_context";
 
-            --  Stable_Sloc is missing a stable backend for C/C++, so default
-            --  to an absolute entry.
+            when C_Language | CPP_Language => SS_Backend := +"clang_context";
+
+            when All_Languages =>
+
+               --  This should not hit but maybe with rust coverage we may
+               --  hit this?
 
-            when All_Languages | C_Language | CPP_Language =>
                SS_Backend := +"absolute";
          end case;
       end if;