[Ada] Create object file for ignored ghost compilation unit

[Pierre-Marie de Rodat <derodat@adacore.com>]

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If the compilation unit we're compiling is an ignored ghost, we need to
ensure that an object file is generated. This happens automatically for
the GCC backend since it always generates the .o file, but for GNAT
LLVM, the backend isn't called. So we need to be sure that we explicitly
generate the file.

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

gcc/ada/

	* gnat1drv.adb (Gnat1drv): Force generation of object file if
	compilation unit is an ignored ghost.

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diff --git a/gcc/ada/gnat1drv.adb b/gcc/ada/gnat1drv.adb
--- a/gcc/ada/gnat1drv.adb
+++ b/gcc/ada/gnat1drv.adb
@@ -1415,18 +1415,19 @@ begin
 
       if Back_End_Mode = Skip then
 
-         --  An ignored Ghost unit is rewritten into a null statement because
-         --  it must not produce an ALI or object file. Do not emit any errors
-         --  related to code generation because the unit does not exist.
+         --  An ignored Ghost unit is rewritten into a null statement. Do
+         --  not emit any errors related to code generation because the
+         --  unit does not exist.
 
          if Is_Ignored_Ghost_Unit (Main_Unit_Node) then
 
             --  Exit the gnat driver with success, otherwise external builders
             --  such as gnatmake and gprbuild will treat the compilation of an
-            --  ignored Ghost unit as a failure. Note that this will produce
-            --  an empty object file for the unit.
+            --  ignored Ghost unit as a failure. Be sure we produce an empty
+            --  object file for the unit.
 
             Ecode := E_Success;
+            Back_End.Gen_Or_Update_Object_File;
 
          --  Otherwise the unit is missing a crucial piece that prevents code
          --  generation.