Revert "Lazyly initialize uncommon toolchain detector"

clang/test/Driver/rocm-detect.hip is failing for a number of
configurations, for example:

clang-x86_64-debian-fast
https://lab.llvm.org/buildbot/#/builders/109/builds/57270

clang-debian-cpp20
https://lab.llvm.org/buildbot/#/builders/249/builds/310

clang-with-lto-ubuntu
https://lab.llvm.org/buildbot/#/builders/124/builds/6693

This reverts commit 6fa2abf9.