[Clang] Export CanPassInRegisters as a type trait

While working on D140664, I thought it would be nice to be able to write tests
for parameter passing ABI. Currently we test this by dumping the AST and
matching the results which makes it hard to write new tests.
Adding this builtin will allow writing better ABI tests which
can help improve our coverage in this area.

While less useful, maybe some users would also find it useful for asserting
against pessimisations for their classes.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D141775