[ModuleInterfaces] Clear dependencies before checking a different module

Sometimes, if one module we tried failed, we would leave the dependencies from the previous check and fail a different module which would have succeeded.

This manifested as the prebuilt-module-cache-forwarding test testing the wrong thing, because it retained a mod-time based dependency in the prebuilt cache, which should have been hash based, and used it.

lib/Frontend/ParseableInterfaceModuleLoader.cpp

@@ -913,7 +913,12 @@ class ParseableInterfaceModuleLoaderImpl {
   bool serializedASTBufferIsUpToDate(
       StringRef path, const llvm::MemoryBuffer &buf,
       SmallVectorImpl<FileDependency> &allDeps) {
-    LLVM_DEBUG(llvm::dbgs() << "Validating deps of " << modulePath << "\n");
+
+    // Clear the existing dependencies, because we're going to re-fill them
+    // in validateSerializedAST.
+    allDeps.clear();
+
+    LLVM_DEBUG(llvm::dbgs() << "Validating deps of " << path << "\n");
     auto validationInfo = serialization::validateSerializedAST(
         buf.getBuffer(), /*ExtendedValidationInfo=*/nullptr, &allDeps);
 
@@ -944,6 +949,13 @@ class ParseableInterfaceModuleLoaderImpl {
       StringRef path, const ForwardingModule &fwd,
       SmallVectorImpl<FileDependency> &deps,
       std::unique_ptr<llvm::MemoryBuffer> &moduleBuffer) {
+
+    // Clear the existing dependencies, because we're going to re-fill them
+    // from the forwarding module.
+    deps.clear();
+
+    LLVM_DEBUG(llvm::dbgs() << "Validating deps of " << path << "\n");
+
     // First, make sure the underlying module path exists and is valid.
     auto modBuf = fs.getBufferForFile(fwd.underlyingModulePath);
     if (!modBuf || !serializedASTLooksValid(*modBuf.get()))