[Hexagon] Make conversions to vector predicate types explicit for builtins

HVX does not have load/store instructions for vector predicates (i.e. bool
vectors). Because of that, vector predicates need to be converted to another
type before being stored, and the most convenient representation is an HVX
vector.
As a consequence, in C/C++, source-level builtins that either take or
produce vector predicates take or return regular vectors instead. On the
other hand, the corresponding LLVM intrinsics do have boolean types that,
and so a conversion of the operand or the return value was necessary.
This conversion would happen inside clang's codegen, but was somewhat
fragile.

This patch changes the strategy: a builtin that takes a vector predicate
now really expects a vector predicate. Since such a predicate cannot be
provided via a variable, this builtin must be composed with other builtins
that either convert vector to a predicate (V6_vandvrt) or predicate to a
vector (V6_vandqrt).

For users using builtins def...