Tests: remove -lstdc++ on Windows test expectations (#5816)

Windows uses a different C++ runtime, which we rely on the linker driver
to adjust the linker invocation (due to ABI incompatibilities between
debug and release builds).  This allows the BuildPlan tests to now pass
on Windows.

Author: compnerd

Tests/BuildTests/BuildPlanTests.swift

@@ -844,7 +844,6 @@ final class BuildPlanTests: XCTestCase {
       #elseif os(Windows)
         XCTAssertEqual(try result.buildProduct(for: "exe").linkArguments(), [
             result.plan.buildParameters.toolchain.swiftCompilerPath.pathString,
-            "-lstdc++",
             "-L", buildPath.pathString,
             "-o", buildPath.appending(components: "exe.exe").pathString,
             "-module-name", "exe",
@@ -1606,7 +1605,7 @@ final class BuildPlanTests: XCTestCase {
 
       #if os(macOS)
         XCTAssertMatch(linkArgs, ["-lc++"])
-      #else
+      #elseif !os(Windows)
         XCTAssertMatch(linkArgs, ["-lstdc++"])
       #endif
     }
@@ -1932,7 +1931,6 @@ final class BuildPlanTests: XCTestCase {
     #elseif os(Windows)
         XCTAssertEqual(try result.buildProduct(for: "lib").linkArguments(), [
             result.plan.buildParameters.toolchain.swiftCompilerPath.pathString,
-            "-lstdc++",
             "-L", buildPath.pathString,
             "-o", buildPath.appending(components: "lib.dll").pathString,
             "-module-name", "lib",