[Ada] Accept compilation switches -Og/-Ofast in non-GCC backends

Tools like GNATprove built as GNAT backends rely on adabkend.adb to
handle generic switches like the optimisation switches -Oxxx.
This patch adds support for -Og and -Ofast that was missing.

There is no impact on compilation.

2019-07-05  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* adabkend.adb (Scan_Back_End_Switches): Accept -Og and -Ofast
	switches.

From-SVN: r273108

gcc/ada/ChangeLog

+2019-07-05  Yannick Moy  <moy@adacore.com>
+
+	* adabkend.adb (Scan_Back_End_Switches): Accept -Og and -Ofast
+	switches.
+
 2019-07-05  Hristian Kirtchev  <kirtchev@adacore.com>
 
 	* ali.adb: Relocate types Invocation_Construct_Record,

gcc/ada/adabkend.adb

@@ -117,9 +117,11 @@ package body Adabkend is
 
          --  Set optimization indicators appropriately. In gcc-based GNAT this
          --  is picked up from imported variables set by the gcc driver, but
-         --  for compilers with non-gcc back ends we do it here to allow use
-         --  of these switches by the front end. Allowed optimization switches
-         --  are -Os (optimize for size), -O[0123], and -O (same as -O1).
+         --  for compilers with non-gcc back ends we do it here to allow use of
+         --  these switches by the front end. Allowed optimization switches are
+         --  -Os (optimize for size), -O[0123], -O (same as -O1), -Ofast
+         --  (disregard strict standards compliance), and -Og (optimize
+         --  debugging experience).
 
          elsif Switch_Chars (First) = 'O' then
             if First = Last then
@@ -134,10 +136,21 @@ package body Adabkend is
                   Optimization_Level :=
                     Character'Pos (Switch_Chars (Last)) - Character'Pos ('0');
 
+               --  Switch -Og is between -O0 and -O1 in GCC. Consider it like
+               --  -O0 for other back ends.
+
+               elsif Switch_Chars (Last) = 'g' then
+                  Optimization_Level := 0;
+
                else
                   Fail ("invalid switch: " & Switch_Chars);
                end if;
 
+            --  Switch -Ofast enables -O3
+
+            elsif Switch_Chars (First + 1 .. Last) = "fast" then
+               Optimization_Level := 3;
+
             else
                Fail ("invalid switch: " & Switch_Chars);
             end if;
@@ -169,7 +182,7 @@ package body Adabkend is
 
             return;
 
-         --  Special check, the back end switch -fno-inline also sets the
+         --  Special check, the back-end switch -fno-inline also sets the
          --  front end flags to entirely inhibit all inlining. So we store it
          --  and set the appropriate flags.
 
@@ -206,7 +219,7 @@ package body Adabkend is
                end case;
             end if;
 
-         --  Ignore all other back end switches
+         --  Ignore all other back-end switches
 
          elsif Is_Back_End_Switch (Switch_Chars) then
             null;