[ParseableInterface] mkdir -p the module cache before using it

Clang does the same thing for its module cache.

lib/Frontend/ParseableInterfaceSupport.cpp

@@ -279,6 +279,7 @@ static bool buildSwiftModuleFromSwiftInterface(
     StringRef ModuleCachePath, DependencyTracker *OuterTracker) {
   bool SubError = false;
   bool RunSuccess = llvm::CrashRecoveryContext().RunSafelyOnThread([&] {
+    (void)llvm::sys::fs::create_directory(ModuleCachePath);
 
     llvm::BumpPtrAllocator SubArgsAlloc;
     llvm::StringSaver SubArgSaver(SubArgsAlloc);

test/ParseableInterface/ModuleCache/module-cache-init.swift

@@ -1,5 +1,4 @@
 // RUN: %empty-directory(%t)
-// RUN: %empty-directory(%t/modulecache)
 //
 // Test will build a module TestModule that depends on OtherModule and LeafModule (built from other.swift and leaf.swift).
 //