Showing file that has sent headers if cannot start session

If session cannot be started because of already-sent headers, show the file and line from which the output has been started.
merged branch davefx/patch-1 (PR #7729)

This PR was merged into the master branch.

Discussion
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If cannot start session due to already-sent headers, show the file and line that have sent them

If session cannot be started because of already-sent headers, show the file and line from which the output has been started.

Commits
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00d2643 Showing file that has sent headers if cannot start session
[HttpFoundation] fixed CS of previous merge