Use CONSTEXPR in machmode.h

Use CONSTEXPR in machmode.h

This patch adds CONSTEXPR to constructors and operators in machmode.h,
excluding the controversial constexpr-resistant "() {}" constructors,
which would become "= default" if we required C++11.

Some of the other routines could potentially be constexpr too,
but that can be done on an as-needed basis.

2019-10-07  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* machmode.h (opt_mode): Mark constructors with CONSTEXPR.
	(pod_mode): Mark operators likewise.
	(scalar_int_mode): Mark non-default constructors and
	operators with CONSTEXPR.
	(scalar_float_mode, scalar_mode, complex_mode): Likewise.
	(fixed_size_mode): Likewise.

From-SVN: r276649