[Ada] Fix references to Backend_Layout configuration parameter

Apparently the Backend_Layout target configuration parameter was renamed to
Frontend_Layout a long time ago (and their meanings are opposite). However,
some comments were still referencing the no longer existing Backend_Layout.
This patch fixes such references.

No test provided, because only comments has been modified.

2018-05-24  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* layout.ads, repinfo.ads: Fix references to renamed Backend_Layout
	configuration parameter.

From-SVN: r260642

gcc/ada/ChangeLog

+2018-05-24  Piotr Trojanek  <trojanek@adacore.com>
+
+	* layout.ads, repinfo.ads: Fix references to renamed Backend_Layout
+	configuration parameter.
+
 2018-05-24  Doug Rupp  <rupp@adacore.com>
 
 	* argv-lynxos178-raven-cert.c: New file.

gcc/ada/layout.ads

@@ -26,7 +26,7 @@
 --  This package does front-end layout of types and objects. The result is
 --  to annotate the tree with information on size and alignment of types
 --  and objects. How much layout is performed depends on the setting of the
---  target dependent parameter Backend_Layout.
+--  target dependent parameter Frontend_Layout.
 
 with Types; use Types;
 
@@ -40,9 +40,9 @@ package Layout is
    procedure Layout_Type (E : Entity_Id);
    --  This procedure may set or adjust the fields Esize, RM_Size and
    --  Alignment in the non-generic type or subtype entity E. If the
-   --  Backend_Layout switch is False, then it is guaranteed that all
+   --  Frontend_Layout switch is True, then it is guaranteed that all
    --  three fields will be properly set on return. Regardless of the
-   --  Backend_Layout value, it is guaranteed that all discrete types
+   --  Frontend_Layout value, it is guaranteed that all discrete types
    --  will have both Esize and RM_Size fields set on return (since
    --  these are static values). Note that Layout_Type is not called
    --  for generic types, since these play no part in code generation,
@@ -53,7 +53,7 @@ package Layout is
    --  a loop parameter (E_Loop_Parameter), or a formal parameter of
    --  a non-generic subprogram (E_In_Parameter, E_In_Out_Parameter,
    --  or E_Out_Parameter). This procedure may set or adjust the
-   --  Esize and Alignment fields of E. If Backend_Layout is False,
+   --  Esize and Alignment fields of E. If Frontend_Layout is True,
    --  then it is guaranteed that both fields will be properly set
    --  on return. If the Esize is still unknown in the latter case,
    --  it means that the object must be allocated dynamically, since

gcc/ada/repinfo.ads

@@ -56,13 +56,13 @@ package Repinfo is
    --       for example in the case where representation clauses or
    --       pragmas specify the values.
 
-   --    2. If Backend_Layout is True, then the backend is responsible
+   --    2. If Frontend_Layout is False, then the backend is responsible
    --       for layout of all types and objects not laid out by the
    --       front end. This includes all dynamic values, and also
    --       static values (e.g. record sizes) when not set by the
    --       front end.
 
-   --    3. If Backend_Layout is False, then the front end lays out
+   --    3. If Frontend_Layout is True, then the front end lays out
    --       all data, according to target dependent size and alignment
    --       information, creating dynamic inlinable functions where
    --       needed in the case of sizes not known till runtime.
@@ -71,7 +71,7 @@ package Repinfo is
    -- Back-Annotation by Gigi --
    -----------------------------
 
-   --  The following interface is used by gigi if Backend_Layout is True
+   --  The following interface is used by gigi if Frontend_Layout is False
 
    --  As part of the processing in gigi, the types are laid out and
    --  appropriate values computed for the sizes and component positions
@@ -209,7 +209,7 @@ package Repinfo is
    -- Front-End Interface for Dynamic Size/Offset Values --
    --------------------------------------------------------
 
-   --  If Backend_Layout is False, then the front-end deals with all
+   --  If Frontend_Layout is True, then the front-end deals with all
    --  dynamic size and offset fields. There are two cases:
 
    --    1. The value can be computed at the time of type freezing, and