Reapply 6fa2abf9

Lazyly initialize uncommon toolchain detector

Cuda and rocm toolchain detectors are currently run unconditionally,
while their result may not be used at all. Make their initialization
lazy so that the discovery code is not run in common cases.

Reapplied since 77910ac3 landed and
fixes the test ordering issue.

Differential Revision: https://reviews.llvm.org/D142606