[CUDA] Detect installation in PATH

If the CUDA toolkit is not installed to its default locations
in /usr/local/cuda, the user is forced to specify --cuda-path.
This is tedious and the driver can be smarter if well-known tools
(like ptxas) can already be found in the PATH environment variable.

Add option --cuda-path-ignore-env if the user wants to ignore
set environment variables. Also use it in the tests to make sure
the driver always finds the same CUDA installation, regardless
of the user's environment.

Differential Revision: https://reviews.llvm.org/D42642

llvm-svn: 323848