[Ada] Suppress GNAT FE up-level reference transformation for GNAT-LLVM

In the case of GNAT-LLVM, the GNAT FE no longer does expansion of
up-level references identified by the subprogram unnesting machinery
into activation record references. This is now only done by the FE when
generating C code. This expansion is already taken care of by the
gnat-llvm middle phase, so there's no benefit to also doing it in the
front end.

2019-09-19  Gary Dismukes  <dismukes@adacore.com>

gcc/ada/

	* exp_unst.adb (Unnest_Subprogram): Bypass the transformation of
	up-level references unless Opt.Generate_C_Code is enabled.

From-SVN: r275942

gcc/ada/ChangeLog

+2019-09-19  Gary Dismukes  <dismukes@adacore.com>
+
+	* exp_unst.adb (Unnest_Subprogram): Bypass the transformation of
+	up-level references unless Opt.Generate_C_Code is enabled.
+
 2019-09-19  Eric Botcazou  <ebotcazou@adacore.com>
 
 	* exp_ch4.adb (Expand_Array_Equality): If optimization is

gcc/ada/exp_unst.adb

@@ -2177,11 +2177,14 @@ package body Exp_Unst is
             --  not need rewriting (e.g. the appearence in a conversion).
             --  Also ignore if no reference was specified or if the rewriting
             --  has already been done (this can happen if the N_Identifier
-            --  occurs more than one time in the tree).
+            --  occurs more than one time in the tree). Also ignore references
+            --  when not generating C code (in particular for the case of LLVM,
+            --  since GNAT-LLVM will handle the processing for up-level refs).
 
             if No (UPJ.Ref)
               or else not Is_Entity_Name (UPJ.Ref)
               or else not Present (Entity (UPJ.Ref))
+              or else not Opt.Generate_C_Code
             then
                goto Continue;
             end if;