Check that the clang override environment variables actually do
something. Note that clang++ is only used to link on non-Darwin
platforms when the interop mode is swift-5.9.

'+' isn't allowed in environment variable names so it's impossible to
set the Clang++ executable at the moment. Replace `+` with `X` so that
we can set `SWIFT_DRIVER_CLANGXX_EXEC` to set the clang++ executable.

Do not always hard-code a macOS triple and pass in explicit stdlib paths to the driver invocations

Some clients still have unversioned prebuilt modules, but SwiftDriver assumed
that the versioned path would always exist for macOS (at least for the purposes
of passing it when compiling for Mac Catalyst).

This change duplicates some logic from the Swift frontend to check for the
existence of the versioned macOS prebuilt modules directory before passing it,
optionally stripping off trailing `.0` version components.

This change also updates the driver so that it passes a prebuilt modules directory
relative to the resource directory rather than relative to the compiler binary, also
matching the Swift frontend's behavior.

This addresses <rdar://problem/136047054>.

(cherry picked from commit b1573ef)

…passing-Catalyst-prebuilt-modules-6.0

[6.0] Only pass a versioned prebuilt-modules for Mac Catalyst if it exists.

Not all generic-"unix" environments have the Gold linker available to
them, and in some cases, the vendor of the toolchain may provide their
own linker. In these cases, the driver should be internally consistent
with the toolchain that it is shipped with.

Now that we have the clang-linker, we can lean on the linker selection
in the clang-linker to determine a default linker. If the clang-linker,
and thus, the swift compiler driver, are part of a specific toolchain,
that clang-linker should be built for that platform with the appropriate
linker defaults set. If someone overrides the linker with
`-use-ld`, we should still honour that, but should otherwise be
consistent with the appropriate toolchain linker.

Fixes: rdar://123061492

🍒[Swift 6.0] Allow overriding clang++ with an environment variable

🍒[Swift 6.0]: Update Generic Unix linker selection

Fixing the number of dashes

[🍒 Swift-6.0.x] fuse-ld only takes one dash

…ibSwiftScan

When building the compiler toolchain, there is currently a race condition which makes it so that the 'libSwiftScan.dylib' is not ready by the time the driver is used to begin compilation of the Swift standard library. For now, disable in-process queries altogether.

On 'main', this is resolved with swiftlang/swift#77606, which seems to be a challenge to backport to 6.0.