libcpp: Use -1 for closed header-unit fd [PR c++/127097]

_cpp_find_header_unit closes the source descriptor after resolving a
header unit, but records zero instead of the established -1 sentinel.
A later textual include through an alternate pathname therefore skips
reopening the source and reads stdin.  Descriptor zero is also a valid
result from open when stdin was previously closed.

Close every valid descriptor and restore -1.  Add a module regression
that imports a header unit and includes the same file through an
alternate spelling.

Assisted-by: Claude Opus 5 (Anthropic)

libcpp/
	PR c++/127097
	* files.cc (_cpp_find_header_unit): Close descriptor zero and set
	the closed descriptor sentinel to -1.

gcc/testsuite/
	PR c++/127097
	* g++.dg/modules/pragma-once-hu-1_a.H: New test.
	* g++.dg/modules/pragma-once-hu-1_b.C: New test.

Signed-off-by: Alexey Klimkin <alexey.klimkin@intel.com>