[LoongArch][clang] Use signed char vectors instead of char vectors for LSX and LASX builtins

`-flax-vector-conversions=none` does not allow an implicit conversion
from `signed char` vector to `char` vector, and we cannot remove `signed`
from `v16i8` or `v32i8` because doing so will break our expectation with
`-fno-signed-char`.  So to make lsxintrin.h and lasxintrin.h work fine
with `-flax-vector-conversions=none`, we must use `signed char` instead
of `char`.

The change is just done via

    sed 's/V16c/V16Sc/g' -i BuiltinsLoongArchLSX.def
    sed 's/V32c/V32Sc/g' -i BuiltinsLoongArchLASX.def

Author: xry111