[Ada] Allow assignment to wide string.

[Ada] Allow assignment to wide string.

Given the following variable declaration...

   Www : Wide_String := "12345";

... this patch allows the following assignment to work:

   (gdb) set variable www := "qwert"

Without this patch, the debugger rejects the assignment because
the size of the array elements are different:

   (gdb) set www := "asdfg"
   Incompatible types in assignment

(on the lhs, we have an array of 2-bytes elements, and on the rhs,
we have a standard 1-byte string).

gdb/ChangeLog:

        * ada-lang.c (ada_same_array_size_p): New function.
        (ada_promote_array_of_integrals): New function.
        (coerce_for_assign): Add handling of arrays where the elements
        are integrals of a smaller size than the size of the target
        array element type.

gdb/testsuite/ChangeLog:

        * gdb.ada/set_wstr: New testcase.