Fix C handling of use of lvalues of incomplete types (PR c/36941, PR c/88827).

Fix C handling of use of lvalues of incomplete types (PR c/36941, PR c/88827).

Bug 88827 points out that GCC should not be rejecting C code that
dereferences a pointer to an incomplete type in the case that uses &*
to take the address of the resulting lvalue, because no constraint is
violated in that case (other than for C90 when the incomplete type is
unqualified void, which we already handle correctly) and as the lvalue
never gets converted to an rvalue there is no undefined behavior
either.

This means that the diagnostic for such a dereference is bogus and
should be removed; if the lvalue gets converted to an rvalue, there
should be an appropriate error later for the use of the incomplete
type.  In most cases, there is, but bug 36941 points out the lack of a
diagnostic when the incomplete (non-void) type gets cast to void
(where a diagnostic seems appropriate for this undefined behavior as a
matter of quality of implementation).

This patch removes the bogus diagnostic (and C_TYPE_ERROR_REPORTED
which was only used in the code that is removed - only that one, bogus
diagnostic had this duplicate suppression, not any of the other, more
legitimate diagnostics for use of incomplete types) and makes
convert_lvalue_to_rvalue call require_complete_type for arguments not
of void types, so that all relevant code paths (possibly except some
for ObjC) get incomplete types diagnosed.  It's possible that this
makes some other checks for incomplete types obsolete, but no attempt
is made to remove any such checks.

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

	PR c/36941
	PR c/88827
gcc/c:
	* c-typeck.c (convert_lvalue_to_rvalue): Call
	require_complete_type for arguments not of void types.
	(build_indirect_ref): Do not diagnose dereferencing pointers to
	incomplete types.
	* c-tree.h (C_TYPE_ERROR_REPORTED): Remove.

gcc/testsuite:
	* gcc.dg/lvalue-9.c, gcc.dg/lvalue-10.c: New tests.
	* gcc.dg/array-8.c, gcc.dg/enum-incomplete-1.c,
	gcc.dg/enum-incomplete-3.c, gcc.dg/noncompile/incomplete-3.c,
	gcc.dg/pr48552-1.c, gcc.dg/pr48552-2.c, gcc.dg/pr63543.c,
	gcc.dg/pr69796.c: Update expected diagnostics.

From-SVN: r278976