[PATCH] Fortran: Fix FMT_X/TR output into already-written content, [PR114618]

[Jerry D]

See attached patch which cleans up I hope the last of the tabbing issues in 
formatted output.  I renamed the existing test case and created a new one to 
test several variations.

Comments appreciated.

Regression tested on X86_64.

Ok for mainline? No plans to backport unless someone thinks its needed.

Best regards,

Jerry

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When an X or TR edit descriptor advances the file position into
content that has already been written (e.g. after a backward T or
TL tab), formatted_transfer_scalar_write either corrupted the
pending_spaces accounting or overwrote the existing characters with
blanks.

Fix this by detecting the case where pending_spaces is negative and
skips is positive (the advance falls within already-written content)
and using write_x (skips, 0) to move the file position forward
without overwriting.  Also, for stream I/O, compute bytes_used from
fbuf->pos (the current write position) rather than fbuf->act (the
high-water mark), since the two diverge after a backward tab followed
by a partial overwrite, which previously prevented this case from
being detected for stream units.

Assisted by: Claude Sonnet 4.6

	PR libfortran/114618

libgfortran/ChangeLog:

	* io/transfer.c (formatted_transfer_scalar_write): Use
	fbuf->pos rather than fbuf->act for bytes_used with stream
	I/O.  When pending_spaces is negative and skips is positive,
	advance the position with write_x (skips, 0) instead of
	overwriting existing content.

gcc/testsuite/ChangeLog:

	* gfortran.dg/pr114618.f90: Move to...
	* gfortran.dg/fmt_t_10.f90: ...here.
	* gfortran.dg/fmt_t_11.f90: New test.
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