[WebAssembly] Use functions instead of macros for const SIMD intrinsics

To improve hygiene, consistency, and usability, it would be good to replace all
the macro intrinsics in wasm_simd128.h with functions. The reason for using
macros in the first place was to enforce the use of constants for some arguments
using `_Static_assert` with `__builtin_constant_p`. This commit switches to
using functions and uses the `__diagnose_if__` attribute rather than
`_Static_assert` to enforce constantness.

The remaining macro intrinsics cannot be made into functions until the builtin
functions they are implemented with can be replaced with normal code patterns
because the builtin functions themselves require that their arguments are
constants.

This commit also fixes a bug with the const_splat intrinsics in which the f32x4
and f64x2 variants were incorrectly producing integer vectors.

Differential Revision: https://reviews.llvm.org/D102018