Allow CHARACTER literals in assignments and data statements.

Allow CHARACTER literals in assignments and data statements.

Allows character literals to used to assign values to non-character variables
in the same way that Hollerith constants are used. In addition character
literals can be used in data statements just like Hollerith constants.

Warnings of such use are output to discourage this usage as it is a non-standard
legacy feature and must be explicitly enabled.

Enabled by -fdec and -fdec-char-conversions.

Co-Authored-By: Jim MacArthur <jim.macarthur@codethink.co.uk>
From-SVN: r277975