Ignore prebuilt module cache when building the prebuilt module cache

When building modules for an SDK, we need to ignore any existing prebuilt
modules to avoid building a partial prebult module cache. Do so by passing
an intentially-bad path for the prebuilt module cache path that's derived
from the output path (but not the same as that path). This prohibits the
frontend scanning job from adding the default prebuilt module cache path,
while ensuring that we find no prebuilt modules.

Fixes rdar://131183526.

Author: DougGregor

Sources/swift-build-sdk-interfaces/main.swift

@@ -157,6 +157,18 @@ do {
         try localFileSystem.createDirectory(mcpPath, recursive: true)
       }
     }
+
+    // When building modules for an SDK,  ignore any existing prebuilt modules.
+    // modules. Do so by passing an intentially-bad path for the prebuilt
+    // module cache path that's derived from the output path (but not the same
+    // as that path). This prohibits the frontend scanning job from adding the
+    // default prebuilt module cache path, while ensuring that we find no
+    // prebuilt modules during this scan.
+    args.append("-Xfrontend")
+    args.append("-prebuilt-module-cache-path")
+    args.append("-Xfrontend")
+    args.append(outputDir.appending(component: "__nonexistent__").pathString)
+
     let baselineABIDir = try getArgumentAsPath("-baseline-abi-dir")
     var driver = try Driver(args: args,
                             diagnosticsOutput: .engine(diagnosticsEngine),