[Cherry-pick] Ensure the macOS SDK exists at install time (#481)

* Ensure the macOS SDK exists at install time (#480)

* Ensure the macOS SDK exists at install time

* update warning messaging

* remove backticks

(cherry picked from commit 20ceb47)

* Fix SDK logic for 1.1

Author: justice-adams-apple

Sources/MacOSPlatform/MacOS.swift

@@ -47,11 +47,32 @@ public struct MacOS: Platform {
     }
 
     public func verifySystemPrerequisitesForInstall(
-        _: SwiftlyCoreContext, platformName _: String, version _: ToolchainVersion,
+        _ ctx: SwiftlyCoreContext, platformName _: String, version _: ToolchainVersion,
         requireSignatureValidation _: Bool
     ) async throws -> String? {
-        // All system prerequisites should be there for macOS
-        nil
+        // Ensure that there is in fact a macOS SDK installed so the toolchain is usable.
+        guard let result = try await self.runProgramOutput(
+            "/usr/bin/xcrun", "--show-sdk-path", "--sdk", "macosx"
+        ) else {
+            // Simply print warnings to the user stdout rather than returning a shell script, as there is not a simple
+            // shell script for installing developer tools on macOS.
+            await ctx.print("""
+            \nWARNING: Could not find a macOS SDK on the system. A macOS SDK is required for the toolchain to work correctly. Please install one via Xcode (https://developer.apple.com/xcode) or run the following command on your machine to install the Command Line Tools for Xcode:
+            xcode-select --install
+
+            More information on installing the Command Line Tools can be found here: https://developer.apple.com/documentation/xcode/installing-the-command-line-tools/#Install-the-Command-Line-Tools-package-in-Terminal. If developer tools are located at a non-default location on disk, use the following command to specify the Xcode that you wish to use for Command Line Tools for Xcode:
+            sudo xcode-select --switch path/to/Xcode.app\n
+            """)
+            return nil
+        }
+
+        let sdkPath = result.replacingOccurrences(of: "\n", with: "")
+
+        if sdkPath == nil {
+            await ctx.print("WARNING: Could not read output of '/usr/bin/xcrun --show-sdk-path --sdk macosx'. Ensure your macOS SDK is installed properly for the swift toolchain to work.")
+        }
+
+        return nil
     }
 
     public func install(