Optimize the degenerate case of a FROM clause table name enclosed all by

Optimize the degenerate case of a FROM clause table name enclosed all by
itself inside parentheses.  Generate code as if the parentheses did not
exist, rather than the old behavior of manifesting the parenthesized table
into a transient table.  Also, tag every FROM-clause SELECT subquery that is
generated by a parenthesized FROM-clause expression using the SF_NestedFrom
flag.  The new SF_NestedFrom flag is not yet used for anything.

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