Implement C11 excess precision semantics for conversions (PR c/82071).

Implement C11 excess precision semantics for conversions (PR c/82071).

C11 semantics for excess precision (from N1531) are that an implicit
conversion (from the usual arithmetic conversions, not by assignment)
from integer to floating point has a result in the corresponding
evaluation format of that floating-point type, so possibly with excess
precision (whereas a cast or conversion by assignment from integer to
floating point must produce a value without excess range or precision,
as always).  This patch makes GCC support those semantics if
flag_isoc11 (which in turn means that conditional expressions need to
support generating a result with excess precision even if neither
operand had excess precision).

C99 is less than entirely clear in this regard, but my reading as
outlined at <https://gcc.gnu.org/ml/gcc-patches/2008-11/msg00105.html>
is that the results of conversions from integer to floating-point
types are always expected to be representable in the target type
without excess precision, and this patch conservatively keeps these
semantics for pre-C11 (i.e. if an older standard is explicitly
selected).

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

	PR c/82071

gcc/c:
	* c-typeck.c (ep_convert_and_check): Just call convert_and_check
	for C11.
	(build_conditional_expr): For C11, generate result with excess
	precision when one argument is an integer and the other is of a
	type using excess precision.

gcc/testsuite:
	* gcc.target/i386/excess-precision-8.c: New test.

From-SVN: r252847