copy stack related elements to str compiler

copy stack related elements to str compiler

Fixed issue in stringify for SQL elements, where a specific dialect is not
passed,  where a dialect-specific element such as the PostgreSQL "on
conflict do update" construct is encountered and then fails to provide for
a stringify dialect with the appropriate state to render the construct,
leading to internal errors.

Fixed issue where stringifying or compiling a :class:`.CTE` that was
against a DML construct such as an :func:`_sql.insert` construct would fail
to stringify, due to a mis-detection that the statement overall is an
INSERT, leading to internal errors.

Fixes: #10753
Change-Id: I783eca3fc7bbc1794fedd325d58181dbcc7e0b75