[Ada] Skip postponed validation checks with compilation errors

[Pierre-Marie de Rodat via Gcc-patches]

If there are compilation errors then gigi doesn't backannotate the AST
with sizes, alignment, etc. The postponed compilation validation checks
can then easily crash or give spurious errors. We now just skip them.

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

gcc/ada/

        * gnat1drv.adb (Gnat1drv): Skip postponed checks when there are
        errors.
diff --git a/gcc/ada/gnat1drv.adb b/gcc/ada/gnat1drv.adb
--- a/gcc/ada/gnat1drv.adb
+++ b/gcc/ada/gnat1drv.adb
@@ -1256,7 +1256,6 @@ begin
 
       if Compilation_Errors then
          Treepr.Tree_Dump;
-         Post_Compilation_Validation_Checks;
          Errout.Finalize (Last_Call => True);
          Errout.Output_Messages;
          Namet.Finalize;