[SR-15135] Copy over early swift-driver executable to the build dir instead of creating a symlink

[artemcm]

The SwiftDriver searches swift-frontend based on Bundle.main.executablePath (which internally uses CFGetProcessPath). This search dir is resolved differently on macOS and Linux so swift-frontend can't be found on Linux, forcing the driver to fallback to using the host system toolchain instead of the just-built one.

[artemcm]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 77436f95bc24e947b0d90ef7ae0fdf147ef3e05a

[finagolfin]

I think this will work, but one problem is that this isn't tested on the linux CI, as there is no prebuilt host toolchain installed on there:

 ('-- Warning: {}', 'Host toolchain could not locate a compiler to build swift-driver. (Try `--skip-early-swift-driver`)')

[artemcm]

@swift-ci please test macOS platform

[artemcm]

I think this will work, but one problem is that this isn't tested on the linux CI, as there is no prebuilt host toolchain installed on there:

 ('-- Warning: {}', 'Host toolchain could not locate a compiler to build swift-driver. (Try `--skip-early-swift-driver`)')

Ah, that's pretty unfortunate.
I've tested this locally on an Ubuntu 20.04 machine with a host toolchain installed and stdlib builds correctly using the just-built compiler.

[artemcm]

@swift-ci please test

[artemcm]

@compnerd does this look okay to merge?

[compnerd]

Yeah, I think that its safe, minor note request, but a follow up is just as fine, no need to wait for the CI times.

[artemcm]

@swift-ci please smoke test