generator/linux: Use --ldpath linker flag for 5.9

Swift Package Manager's linker flag handling changed between
5.9 and 5.10.   The flags which work for 5.9 cause linking failures
5.10 and later, and vice versa:

  swiftlang/swift-package-manager#7222

This commit generates workaround flags for 5.9 and new-style flags
for all other versions.

The EndToEnd tests currently cannot run in CI, so this change was
tested locally. (Issue swiftlang#145)

* The basic 'hello world' example generated by `swift package init`
  built successfully with 5.9.2, 5.10.1 and 6.0.2 SDKs on x86_64 and aarch64.
* A more complex example using Vapor built succesfully with 5.9.2, 5.10.1.  6.0.2
  failed because of the CShims problem reported in Issue swiftlang#138.

Sources/SwiftSDKGenerator/SwiftSDKRecipes/LinuxRecipe.swift

@@ -105,13 +105,20 @@ public struct LinuxRecipe: SwiftSDKRecipe {
   }
 
   public func applyPlatformOptions(toolset: inout Toolset, targetTriple: Triple) {
-    toolset.swiftCompiler = Toolset.ToolProperties(extraCLIOptions: [
-      "-use-ld=lld",
-      "-Xlinker",
-      "-R/usr/lib/swift/linux/",
-    ])
+    var swiftCompilerOptions = ["-Xlinker", "-R/usr/lib/swift/linux/"]
+
+    // Swift 5.9 does not handle the `-use-ld` option properly:
+    //   https://github.com/swiftlang/swift-package-manager/issues/7222
+    if self.versionsConfiguration.swiftVersion.hasPrefix("5.9") {
+      swiftCompilerOptions += ["-Xclang-linker", "--ld-path=ld.lld"]
+    } else {
+      swiftCompilerOptions.append("-use-ld=lld")
+      toolset.linker = Toolset.ToolProperties(path: "ld.lld")
+    }
+
+    toolset.swiftCompiler = Toolset.ToolProperties(extraCLIOptions: swiftCompilerOptions)
+
     toolset.cxxCompiler = Toolset.ToolProperties(extraCLIOptions: ["-lstdc++"])
-    toolset.linker = Toolset.ToolProperties(path: "ld.lld")
     toolset.librarian = Toolset.ToolProperties(path: "llvm-ar")
   }