[VE] Add clang tests for VE

Summary:
Add a preprocessor test to check VE predefinitions.  Add a driver test
to check VE toolchain behavior.

Reviewers: simoll, k-ishizaka

Reviewed By: simoll

Subscribers: krytarowski, jfb, ormris, cfe-commits

Tags: #llvm, #ve, #clang

Differential Revision: https://reviews.llvm.org/D82461