c: Fix boolean conversion of floating constant as integer constant expression [PR113776]

My fix for bug 111059 and bug 111911 caused a conversion of a floating
constant to boolean to wrongly no longer be considered an integer
constant expression, because logic to insert a NOP_EXPR in
c_objc_common_truthvalue_conversion for an argument not an integer
constant expression itself now took place after rather than before the
conversion to bool.  In the specific case of casting a floating
constant to bool, the result is an integer constant expression even
though the argument isn't (build_c_cast deals with ensuring that casts
to integer type of anything of floating type more complicated than a
single floating constant don't get wrongly treated as integer constant
expressions even if they fold to constants), so fix the logic in
c_objc_common_truthvalue_conversion to handle that special case.

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

	PR c/113776

gcc/c
	* c-typeck.cc (c_objc_common_truthvalue_conversion): Return an
	integer constant expression for boolean conversion of floating
	constant.

gcc/testsuite/
	* gcc.dg/pr113776-1.c, gcc.dg/pr113776-2.c, gcc.dg/pr113776-3.c,
	gcc.dg/pr113776-4.c: New tests.