Use `clang` rather than `clang++` as the linker driver. This ensures
that we do not force a C++ runtime on the general code. This is fine
for now as C++ interop is not yet available for Swift. This prevents
the accidental mix-and-match of various C++ runtimes. This can cause
problems on platforms like android where `libstdc++` is an unsupported
runtime but is generally the default for Linux platforms.
Rather than aborting due to an assertion failure, emit a diagnostic.
This is much safer and generally easier to understand why the command
failed. It solves the problem of running swiftc from the build without
the path being set such that the clang++ driver is found by the swift
driver.
