Defer Ada character literal resolution

Defer Ada character literal resolution

In Ada, an enumeration type can use a character literal as one of the
enumerators.  The Ada expression parser handles the appropriate
conversion.

It turns out, though, that this conversion was handled incorrectly.
For an expression like TYPE'(EXP), the conversion would be done for
any such literal appearing in EXP -- but only the outermost such
expression should really be affected.

This patch defers the conversion until the resolution phase, fixing
the bug.