pcm/Clamp: convert to `constexpr`

src/pcm/Clamp.hxx

@@ -34,8 +34,7 @@ template<SampleFormat F> struct SampleTraits;
  * and caps it if necessary.
  */
 template<SampleFormat F, class Traits=SampleTraits<F>>
-gcc_const
-static inline typename Traits::value_type
+constexpr typename Traits::value_type
 PcmClamp(typename Traits::long_type x) noexcept
 {
 	typedef typename Traits::value_type T;

src/pcm/FloatConvert.hxx

@@ -37,8 +37,7 @@ struct FloatToIntegerSampleConvert {
 	static constexpr SV factor = uintmax_t(1) << (DstTraits::BITS - 1);
 	static_assert(factor > 0, "Wrong factor");
 
-	gcc_const
-	static DV Convert(SV src) noexcept {
+	static constexpr DV Convert(SV src) noexcept {
 		return PcmClamp<F, Traits>(SL(src * factor));
 	}
 };

src/pcm/PcmMix.cxx

@@ -134,7 +134,7 @@ pcm_add_vol(PcmDither &dither, void *buffer1, const void *buffer2, size_t size,
 }
 
 template<SampleFormat F, class Traits=SampleTraits<F>>
-static typename Traits::value_type
+static constexpr typename Traits::value_type
 PcmAdd(typename Traits::value_type _a, typename Traits::value_type _b) noexcept
 {
 	typename Traits::sum_type a(_a), b(_b);