If a tool isn't found next to the driver, try looking for it next to the frontend

This might reduce the need for SWIFT_DRIVER_*_EXEC overrides in library clients like SPM

Author: owenv

Sources/SwiftDriver/Toolchains/Toolchain.swift

@@ -126,6 +126,12 @@ extension Toolchain {
       return try AbsolutePath(validating: overrideString)
     } else if let path = lookupExecutablePath(filename: executable, searchPaths: [executableDir]) {
       return path
+    } else if !["swift-frontend", "swift"].contains(executable),
+              let parentDir = try? getToolPath(.swiftCompiler).parentDirectory,
+              parentDir != executableDir,
+              let path = lookupExecutablePath(filename: executable, searchPaths: [parentDir]) {
+      // If the frontend and driver have different parent directories, try looking in both.
+      return path
     } else if let path = try? xcrunFind(executable: executable) {
       return path
     } else if let path = lookupExecutablePath(filename: executable, searchPaths: searchPaths) {