Fix the legacy driver path construction on non-Windows platforms

`swift-driver` tries to invoke `swiftc-legacy-driver.` (trailing dot)
on non-Windows platforms when legacy driver is requested since
#1746

This is because `URL.pathExtension` returns an empty string if the
URL does not have an extension but `URL.appendPathExtension` appends
a period before the extension even if given an empty string.
Thus `url.deletingPathExtension().appendingPathExtension(url.pathExtension)`
will not be equal to `url` if `url` does not have an extension.

Author: kateinoigakukun

Sources/swift-driver/main.swift

@@ -83,10 +83,16 @@ do {
     // invocation for a multi-call binary.
     let executable: URL = URL(fileURLWithPath: ProcessInfo.processInfo.arguments[0])
     let invocation: URL = URL(fileURLWithPath: CommandLine.arguments[0])
-    let legacyExecutablePath: URL =
+    var legacyExecutablePath: URL =
         executable.deletingLastPathComponent()
                   .appendingPathComponent("\(invocation.deletingPathExtension().lastPathComponent)-legacy-driver")
-                  .appendingPathExtension(executable.pathExtension)
+    // Check if the original path has a non-empty extension because
+    // `url.deletingPathExtension().appendingPathExtension(url.pathExtension)`
+    // will not be equal to `url` but be `\(url).` if `url` doesn't have an
+    // extension.
+    if !executable.pathExtension.isEmpty {
+        legacyExecutablePath.appendPathExtension(executable.pathExtension)
+    }
     let path: String = legacyExecutablePath.withUnsafeFileSystemRepresentation { String(cString: $0!) }
 
     if localFileSystem.exists(AbsolutePath(path)) {