For hosts that have a swiftly-managed Swift compiler, we could not
bootstrap using those tools, because of some hardcoded assumptions
about where the `/lib` directory lives, relative to the `/bin`
directory that contains the detected `swiftc`.
This patch adds specific support for detecting when the `swiftc`
is coming from a swiftly install and uses the correct paths.
I've tested this patch on my Linux machine that has swiftly 1.0.1,
with the Swift 6.1.2 toolchain.