[Clang] Fix HIP tests when running on Windows with the LLVM toolchain is in the path

On Windows, when the LLVM toolchain is in the current path (%PATH%), fusing the linker yields c:\{path}\lld.exe whereas the hip tests did not expect the .exe part.
This only happens if LLD is not present in the build folder, which consequently makes the code in Driver::GetProgramPath() to fall back to %PATH% and platform-specific search, which includes the .exe part (see https://github.com/llvm/llvm-project/blob/master/clang/lib/Driver/Driver.cpp#L4733).

Differential Revision: https://reviews.llvm.org/D76631