[Ada] Make string_char_type a true TYPE_CODE_CHAR type in Ada

[Ada] Make string_char_type a true TYPE_CODE_CHAR type in Ada

Before this change, trying to call an overloaded function with at least
one character literal in argument would fail.  For instance, given these
two functions:

   function F (C : Character) return Integer is
   begin
      return Character'Pos (C);
   end F;

   function F (I : Integer) return Integer is
   begin
      return -I;
   end F;

We would get the following GDB session:

    (gdb) p f('A')
    $1 = -65
    (gdb) p f(1)
    $1 = -1

This is wrong because the first call should select the first F function
and thus return 65.

The root problem is that ada-lang.c:ada_language_arch_info stores in
string_char_type a type whose code is TYPE_CODE_INT instead of
TYPE_CODE_CHAR.  As a result, all parsed character literals are turned
into integer values and during overload matching, the TYPE_CODE_CHAR
formal rejects the TYPE_CODE_INT actual.

This change turns string_char_type into a true TYPE_CODE_CHAR type in
ada-lang.c so that we have instead the expected:

    (gdb) p f('A')
    $1 = 65

gdb/ChangeLog:

	* ada-lang.c (ada_language_arch_info): Create a TYPE_CODE_CHAR
	type instead of a TYPE_CODE_INT one for the string_char_type
	and the ada_primitive_type_char types.

gdb/testsuite/ChangeLog:

	* gdb.ada/funcall_char.exp: New testcase.
	* gdb.ada/funcall_char/foo.adb: New file.

Tested on x86_64-linux, no regression.