Add Cxx interop swift settings to CI (#2999)

### Motivation:

At the moment the Cxx interoperability CI workflow doesn't actually test
Cxx interoperability.

### Modifications:

The script now looks for the precise way that the `Package.swift`
currently formats the target entry and appends a `swiftSettings` entry
to it.

This approach is pretty brittle but has the advantage that it's the same
on Linux and Darwin. If this formatting changes too often then we could
give up on this and assume the availability of gnu-sed instead (and make
sure it's present on our CI containers).

### Result:

All Cxx interoperability checks will check what they are named for.

---------

Co-authored-by: George Barnett <[email protected]>

scripts/check-cxx-interop-compatibility.sh

@@ -19,7 +19,7 @@ log() { printf -- "** %s\n" "$*" >&2; }
 error() { printf -- "** ERROR: %s\n" "$*" >&2; }
 fatal() { error "$@"; exit 1; }
 
-log "Checking for Cxx interoperability comaptibility..."
+log "Checking for Cxx interoperability compatibility..."
 
 source_dir=$(pwd)
 working_dir=$(mktemp -d)
@@ -30,6 +30,12 @@ package_name=$(swift package dump-package | jq -r '.name')
 
 cd "$working_dir"
 swift package init
+
+{
+  echo "let swiftSettings: [SwiftSetting] = [.interoperabilityMode(.Cxx)]"
+  echo "for target in package.targets { target.swiftSettings = swiftSettings }"
+} >> Package.swift
+
 echo "package.dependencies.append(.package(path: \"$source_dir\"))" >> Package.swift
 
 for product in $library_products; do
@@ -39,4 +45,4 @@ done
 
 swift build
 
-log "✅ Passed the Cxx interoperability tests."
+log "✅ Passed the Cxx interoperability tests."