[Ada] Improve warnings about "too few elements" and "too many elements"

[Ada] Improve warnings about "too few elements" and "too many elements"

When warning about length-check failures detected at compile time that
are flagged with "too few elements" or "too many elements", the compiler
now gives an additional message indicating the number of elements
expected by the context versus how many are present in the conflicting
expression (such as an aggregate that has too few or too many
components).

The test below reports the following warnings when compiled with this command:

$ gcc -c -gnatj78 length_warnings.adb

length_warnings.adb:6:09: warning: too few elements for subtype of
                          "Boolean_Array" defined at line 5, expected 10
                          elements; found 9 elements, "Constraint_Error" will
                          be raised at run time
length_warnings.adb:10:09: warning: too few elements for subtype of
                           "Boolean_Array" defined at line 9, expected 2
                           elements; found 1 element, "Constraint_Error" will
                           be raised at run time
length_warnings.adb:14:09: warning: too many elements for subtype of
                           "Boolean_Array" defined at line 13, expected 10
                           elements; found 11 elements, "Constraint_Error"
                           will be raised at run time
length_warnings.adb:18:09: warning: too many elements for subtype of
                           "Boolean_Array" defined at line 17, expected 0
                           elements; found 1 element, "Constraint_Error" will
                           be raised at run time
length_warnings.adb:22:09: warning: too many elements for subtype of
                           "Boolean_Array" defined at line 21, expected 1
                           element; found 2 elements, "Constraint_Error" will
                           be raised at run time

procedure Length_Check_Warnings is

   type Boolean_Array is array (Natural range <>) of Boolean;

   Bits_A : Boolean_Array (1 .. 10)
     := (True, True, True, True, True, True, True, True, True);
   -- Too few elements

   Bits_B : Boolean_Array (1 .. 2)
     := (1 => False);
   -- Too few elements

   Bits_C : Boolean_Array (1 .. 10)
     := (True, True, True, True, True, True, True, True, True, True, True);
   -- Too many elements

   Bits_D : Boolean_Array (1 .. 0)
     := (1 => True);
   -- Too many elements

   Bits_E : Boolean_Array (1 .. 1)
     := (True, False);
   -- Too many elements

begin
   null;
end Length_Check_Warnings;

2019-08-19  Gary Dismukes  <dismukes@adacore.com>

gcc/ada/

	* checks.adb (Length_Mismatch_Info_Message): New function in
	Selected_Length_Checks to return a message indicating the
	element counts for the mismatched lengths for a failed
	compile-time length check.
	(Plural_Or_Singular_Ending): Support function in
	Length_Mismatch_Info_Message to return either "" or "s", for
	concatenating to the end of words.
	(Selected_Length_Checks): Pass the result of
	Length_Mismatch_Info_Message as an extra warning message to
	Compile_Time_Constraint_Error to indicate the mismatched lengths
	for a failed compile-time length check.
	* sem_util.ads (Compile_Time_Constraint_Error): Add an optional
	message formal (Extra_Msg), defaulted to the empty string.
	* sem_util.adb (Compile_Time_Constraint_Error): Output an extra
	message following the main warning message (when Extra_Msg is
	not the empty string).

From-SVN: r274652