[Testing] Do not assume unqualified path to clang++

[davezarzycki]

Without this change, unified builds are broken on my Linux workstation (Fedora 30, x86-64).

[davezarzycki]

@swift-ci please smoke test and merge

[compnerd]

@davezarzycki, I believe that the path should be unqualified, as that is what the test is testing. The point here is to ensure that we explicitly are passing clang++ and relying on PATH to find the tool.

[davezarzycki]

I don't think my setup is doing anything weird here, and the fully qualified path made its way through to the test. How can I help you debug this?

[jrose-apple]

I think someone else changed the search to find files in the same directory as the compiler, to make it easier to build full custom toolchain setups. That means this test will never work for a unified build. Maybe it can hard-link the compiler to another location, like some of the other tests in the Driver/ folder?