Handle SubpBody's DefiningName self reference in P_Gnat_Xref

When resolving a subprogram body, the subprogram body's name xrefs
contain a reference to itself. This patch adds that case to
`P_Gnat_Xref` in order to mimic GNAT's behavior.

ada/ast.py

@@ -1663,6 +1663,9 @@ def gnat_xref():
                 )
             ),
 
+            bd.then(lambda bd: bd.is_a(T.BaseSubpBody)),
+            bd.cast(T.BaseSubpBody).subp_spec.subp_name,
+
             Entity.cast(T.Name)._.gnat_xref_decl.then(
                 lambda ret:
                 Let(lambda dbd=ret.basic_decl: Cond(

testsuite/tests/name_resolution/gnat_xref_1/prj.gpr

@@ -0,0 +1,4 @@
+project Prj is
+  for Source_Dirs use (".");
+  for Main use ("test.adb");
+end Prj;

testsuite/tests/name_resolution/gnat_xref_1/test.adb

@@ -0,0 +1,4 @@
+procedure P is
+begin
+   null;
+end P;

testsuite/tests/name_resolution/gnat_xref_1/test.out

@@ -0,0 +1,13 @@
+test.adb:1:11: warning: file name does not match unit name, should be "p.adb" [enabled by default]
+== test.adb ==
+test.adb:1:11 => test.adb:1:11 (LAL: ok)
+test.adb:4:5 => test.adb:1:11 (LAL: ok)
+
+Stats:
+GNAT xrefs have 2 entries
+LAL xrefs have:
+  * 2 OK entries (100.00%)
+  * 0 DIFFERENT entries (0.00%)
+  * 0 ERROR entries (0.00%)
+  * 0 MISSING entries (0.00%)
+  * 0 ADDITIONAL entries (0.00%)

testsuite/tests/name_resolution/gnat_xref_1/test.yaml

@@ -0,0 +1,2 @@
+driver: gnat-compare
+project_file: prj.gpr