Type traits: No need for switch to handle __builtin_types_compatible_p

__builtin_types_compatible_p() isn't a C++ type trait at all, rather a GNU C
special-case, so it's fine to use BoolTy the default return type for binary
type traits.

This brings BTT in line with other arities that already default to BoolTy.

Cleanup only, no change in behaviour.

llvm-svn: 196646