llvm_tools: Validate chromeos checkout path

Add support for verifying chromeos checkout path so
invoking a bad path gives an error immediately.

BUG=b:187794508
TEST=unit tests

Change-Id: I889c2e83540e0ae23680b0480ff71905ac7723fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/4011203
Reviewed-by: Bob Haarman <inglorion@chromium.org>
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Ryan Beltran <ryanbeltran@chromium.org>
Commit-Queue: Manoj Gupta <manojgupta@chromium.org>
11 files changed