[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