This fixes parsing failures, like e.g.
warn:legacy.osl:30429:1:dbaccess/source/core/api/SingleSelectQueryComposer.cxx:783: OSingleSelectQueryComposer::getColumns: could not parse the column retrieval statement!
when opening query "Balance" of https://bugs.freedesktop.org/attachment.cgi?id=71964

OTOH, since we have reduce/reduce conflicts, we might have true
ambiguities in our grammar. This change makes the parsing fail
in the presence of full-expression ambiguities instead of making
a ~arbitrary choice.

Change-Id: I88574da91045dd52f198f193c78d144302f298a4