[clang][NFC] refactors value type traits so we can have more than bools

Since all the type traits up until now have had Boolean vaules, we've
always been able to assume that the expressions are `bool`. This is
about to change (D151952 introduces a trait that returns `size_t`), so
we need to restructure the code so it doesn't become unwieldy.

This is achieved by giving traits a designated "return" type.

Differential Revision: https://reviews.llvm.org/D152034