Un-revert "[Option] Fix PR37006 prefix choice in findNearest"

Summary:
In https://reviews.llvm.org/rL332804 I loosed the assertion in
the Clang driver test that forced me to revert
https://reviews.llvm.org/rL332299. Once this lands I should be
able to narrow down what caused PS4 buildbots to fail, and
reinstate the check in that test.

Test Plan: check-llvm & check-clang

llvm-svn: 332805