Allow re-assigning to convenience variables

In Ada mode, re-assigning an array of a different size to a
convenience variable will cause an error:

    (gdb) set lang ada
    (gdb) set $v := "abc"
    (gdb) set $v := "abcd"
    cannot assign arrays of different length

However, this does not really make sense -- instead, it should always
be possible to overwrite a convenience variable.

This patch fixes this bug.

This was reviewed off-list by Joel.  I'm checking it in.

gdb/ChangeLog
2019-06-14  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (ada_evaluate_subexp) <case BINOP_ASSIGN>: Always
	allow assignment to an internalvar.

gdb/testsuite/ChangeLog
2019-06-14  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/set_wstr.exp: Add reassignment test.