Turn off the is_literal flag when proxying literal_column() to Label

Turn off the is_literal flag when proxying literal_column() to Label

Fixed a series of quoting issues which all stemmed from the concept of the
:func:`.literal_column` construct, which when being "proxied" through a
subquery to be referred towards by a label that matches its text, the label
would not have quoting rules applied to it, even if the string in the
:class:`.Label` were set up as a :class:`.quoted_name` construct.  Not
applying quoting to the text of the :class:`.Label` is a bug because this
text is strictly a SQL identifier name and not a SQL expression, and the
string should not have quotes embedded into it already unlike the
:func:`.literal_column` which it may be applied towards.   The existing
behavior of a non-labeled :func:`.literal_column` being propagated as is on
the outside of a subquery is maintained in order to help with manual
quoting schemes, although it's not clear if valid SQL can be generated for
such a construct in any case.

Fixes: #4730
Change-Id: I300941f27872fc4298c74a1d1ed65aef1a5cdd82