[PSV] Update API to be able to use TargetCustom without UB.

getTargetCustom() requires values for "Kind" in the constructor
that are not in the PSVKind enum. Passing a value that is not inside
an enum as an argument to a constructor of the type of the enum is
UB. Changing to the underlying type of the enum would solve the UB

Differential Revision: https://reviews.llvm.org/D50909

llvm-svn: 340200