Code review and test cases changes

This commit if added will
1. Removes redundant isDarwin check for
 -static-stdlib DarwinToolchain arg validation ,
2. Adds Darwin platform target for all 3 tests introduced,

Author: srinikhil-07

Sources/SwiftDriver/Toolchains/DarwinToolchain.swift

@@ -159,7 +159,7 @@ public final class DarwinToolchain: Toolchain {
       throw ToolchainValidationError.unsupportedTargetVariant(variant: targetVariantTriple)
     }
     // Validating darwin unsupported -static-stdlib argument.
-    if parsedOptions.hasArgument(.staticStdlib) && targetTriple.isDarwin {
+    if parsedOptions.hasArgument(.staticStdlib) {
         throw ToolchainValidationError.argumentNotSupported("-static-stdlib")
     }
     // If a C++ standard library is specified, it has to be libc++.

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -1541,22 +1541,22 @@ final class SwiftDriverTests: XCTestCase {
       }
     }
 
-    XCTAssertThrowsError(try Driver(args: ["swiftc", "-c", "-static-stdlib",
+    XCTAssertThrowsError(try Driver(args: ["swiftc", "-c", "-static-stdlib", "-target", "x86_64-apple-macosx10.14",
                                            "foo.swift"])) { error in
       guard case DarwinToolchain.ToolchainValidationError.argumentNotSupported("-static-stdlib") = error else {
         XCTFail()
         return
       }
     }
 
-    XCTAssertThrowsError(try Driver(args: ["swiftc", "-c", "-experimental-cxx-stdlib", "libstdc++",
+    XCTAssertThrowsError(try Driver(args: ["swiftc", "-c", "-target", "x86_64-apple-macosx10.14", "-experimental-cxx-stdlib", "libstdc++",
                                            "foo.swift"])) { error in
         guard case DarwinToolchain.ToolchainValidationError.darwinOnlySupportsLibCxx = error else {
         XCTFail()
         return
       }
     }
-    try assertNoDriverDiagnostics(args: "swiftc", "-c", "-link-objc-runtime", "foo.swift")
+    try assertNoDriverDiagnostics(args: "swiftc", "-c", "-target", "x86_64-apple-macosx10.14", "-link-objc-runtime", "foo.swift")
   }
 
   func testDSYMGeneration() throws {