Fix Ada integer literals with exponents

While working on another patch, I noticed that Ada integer literals
with exponents did not work.  For example, with one form you get an
error:

    (gdb) print 8e2
    Invalid digit `e' in based literal

And with another form you get an incorrect value:

    (gdb) print 16#8#e2
    $2 = 8

This patch fixes the bugs and adds tests.