Fix use of extraCPPFlags

Since we are using `swiftc` as the linker driver, we need to prefix any C specifc arguments with `-Xcc`.

rdar://100575898

Author: neonichu

Sources/Build/BuildPlan.swift

@@ -2080,7 +2080,8 @@ public class BuildPlan {
         // Note: This will come from build settings in future.
         for target in dependencies.staticTargets {
             if case let target as ClangTarget = target.underlyingTarget, target.isCXX {
-                buildProduct.additionalFlags += self.buildParameters.toolchain.extraCPPFlags
+                // Since we are using `swiftc` as the linker driver, we need to prefix these.
+                buildProduct.additionalFlags += self.buildParameters.toolchain.extraCPPFlags.flatMap({ ["-Xcc", $0] })
                 break
             }
         }

Tests/BuildTests/BuildPlanTests.swift

@@ -1906,7 +1906,7 @@ final class BuildPlanTests: XCTestCase {
     #if os(macOS)
         XCTAssertEqual(try result.buildProduct(for: "lib").linkArguments(), [
             result.plan.buildParameters.toolchain.swiftCompilerPath.pathString,
-            "-lc++",
+            "-Xcc", "-lc++",
             "-L", buildPath.pathString,
             "-o", buildPath.appending(components: "liblib.dylib").pathString,
             "-module-name", "lib",