[Ada] Reset Reachable field when mutating label into loop entity

[Pierre-Marie de Rodat via Gcc-patches]

An entity flag Reachable now only applies to E_Label entities and needs
to be explicitly reset when mutating labels into loop entities. Only
needed to prevent cascaded errors when compiling a malicious ACATS test
with -gnatq (try semantics, even if parse errors).

Cleanup related to detection of uninitialized scalars with GOTO
statements.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * sem_ch5.adb (Analyze_Statements): Only clear Reachable flag on
        proper label entities.
diff --git a/gcc/ada/sem_ch5.adb b/gcc/ada/sem_ch5.adb
--- a/gcc/ada/sem_ch5.adb
+++ b/gcc/ada/sem_ch5.adb
@@ -4012,6 +4012,7 @@ package body Sem_Ch5 is
 
             if Ekind (Ent) = E_Label then
                Reinit_Field_To_Zero (Ent, F_Enclosing_Scope);
+               Reinit_Field_To_Zero (Ent, F_Reachable);
                Mutate_Ekind (Ent, E_Loop);
 
                if Nkind (Parent (Ent)) = N_Implicit_Label_Declaration then