[WebAssembly] Use "signed char" instead of "char" in SIMD intrinsics.

This allows people to use `int8_t` instead of `char`, -funsigned-char,
and generally decouples SIMD from the specialness of `char`.

And it makes intrinsics like `__builtin_wasm_add_saturate_s_i8x16`
and `__builtin_wasm_add_saturate_u_i8x16` use signed and unsigned
element types, respectively.

Differential Revision: https://reviews.llvm.org/D85074