c: Fix up integer promotion of bit-precise bit-fields [PR127383]

The following testcases are miscompiled when a bit-precise integer
bit-field is explicitly cast to a non-cv-qualified bit-precise integer
(of the same signedness) with precision of the bit-field width.
We can then incorrectly apply an integer promotion of the bit-field
to the corresponding bit-precise integer, even when it shouldn't be
applied (because it is a cast rather than the bit-field itself).
Regardless of the cv-qualifications, the TREE_TYPE of the FIELD_DECLs
(and COMPONENT_REFs) is a cv-unqualified BITINT_TYPE.  We first call
convert to that type in convert_lvalue_to_rvalue and that doesn't change
anything.
If there is an explicit cast to the same type as the TREE_TYPE of
the COMPONENT_REF has, we usually wrap it into a NON_LVALUE_EXPR, because
the COMPONENT_REF is usually an lvalue (but as can be seen in the testcases,
not always).  So the second hunk makes sure to add NON_LVALUE_EXPR around
the COMPONENT_REF even in the (cast) foo ().a cases to distinguish it
from foo ().a.
The second change is in default_conversion, for the promotion of bit-fields
of bit-precise integer type we don't promote them if they are wrapped in
some cast or NON_LVALUE_EXPR (the function normally uses STRIP_TYPE_NOPS).

2026-09-16  Jakub Jelinek  <jakub@redhat.com>

	PR c/127383
	* c-typeck.cc (default_conversion): If orig_expr is different
	from exp and exp is a bit-field of bit-precise integer type, call
	perform_integral_promotions on orig_exp rather than exp.
	(build_c_cast): Wrap in NON_LVALUE_EXPR not just lvalues, but
	also non-lvalue bit-fields of bit-precise integer type.

	* gcc.dg/torture/bitint-108.c: New test.
	* gcc.dg/bitint-143.c: New test.
	* gcc.dg/bitint-144.c: New test.

Reviewed-by: Joseph Myers <josmyers@redhat.com>