[clang][test] Use a physical copy of FS

[kadircet]

Summary

Make use of a physical copy, rather than real FS in unittests that
change working-directory to get rid of the side effect of changing cwd for the
whole process. It's triggering crashes depending on the test order.

[kadircet]

• Fix test

[goncharov]

nice

clang/unittests/Serialization/ModuleCacheTest.cpp

 //===- unittests/Serialization/ModuleCacheTest.cpp - CI tests -------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "clang/Basic/FileManager.h"
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Frontend/CompilerInvocation.h"
 #include "clang/Frontend/FrontendActions.h"
 #include "clang/Frontend/Utils.h"
 #include "clang/Lex/HeaderSearch.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/Support/FileSystem.h"
+#include "llvm/Support/VirtualFileSystem.h"
 #include "llvm/Support/raw_ostream.h"
 
 #include "gtest/gtest.h"
 
 using namespace llvm;
 using namespace clang;
 
 namespace {
TEST_F(ModuleCacheTest, CachedModuleNewPath) {
   addDuplicateFrameworks();
 
   SmallString<256> MCPArg("-fmodules-cache-path=");
   MCPArg.append(ModuleCachePath);
   IntrusiveRefCntPtr<DiagnosticsEngine> Diags =
       CompilerInstance::createDiagnostics(new DiagnosticOptions());
   CreateInvocationOptions CIOpts;
   CIOpts.Diags = Diags;
+  CIOpts.VFS = llvm::vfs::createPhysicalFileSystem();
 
   // First run should pass with no errors
   const char *Args[] = {"clang",        "-fmodules",          "-Fframeworks",
                         MCPArg.c_str(), "-working-directory", TestDir.c_str(),
                         "test.m"};
   std::shared_ptr<CompilerInvocation> Invocation =
       createInvocation(Args, CIOpts);
   ASSERT_TRUE(Invocation);

clang/unittests/Serialization/NoCommentsTest.cpp

void foo() {}
   )cpp");
 
   IntrusiveRefCntPtr<DiagnosticsEngine> Diags =
       CompilerInstance::createDiagnostics(new DiagnosticOptions());
   CreateInvocationOptions CIOpts;
   CIOpts.Diags = Diags;
 
   std::string CacheBMIPath = llvm::Twine(TestDir + "/Comments.pcm").str();
-  const char *Args[] = {
-      "clang++",       "-std=c++20",    "--precompile", "-working-directory",
-      TestDir.c_str(), "Comments.cppm", "-o",           CacheBMIPath.c_str()};
+  std::string CCPMPath = llvm::Twine(TestDir).concat("/Comments.cppm").str();
+  const char *Args[] = {"clang++",        "-std=c++20", "--precompile",
+                        CCPMPath.c_str(), "-o",         CacheBMIPath.c_str()};
   std::shared_ptr<CompilerInvocation> Invocation =
       createInvocation(Args, CIOpts);
   ASSERT_TRUE(Invocation);
 
   CompilerInstance Instance;
   Instance.setDiagnostics(Diags.get());
   Instance.setInvocation(Invocation);
   GenerateModuleInterfaceAction Action;

clang/unittests/Serialization/VarDeclConstantInitTest.cpp

export namespace Fibonacci
   template constexpr unsigned long Cache<10ul>;
 }
   )cpp");
 
   IntrusiveRefCntPtr<DiagnosticsEngine> Diags =
       CompilerInstance::createDiagnostics(new DiagnosticOptions());
   CreateInvocationOptions CIOpts;
   CIOpts.Diags = Diags;
+  CIOpts.VFS = llvm::vfs::createPhysicalFileSystem();
 
   std::string CacheBMIPath = llvm::Twine(TestDir + "/Cached.pcm").str();
   const char *Args[] = {
       "clang++",       "-std=c++20",  "--precompile", "-working-directory",
       TestDir.c_str(), "Cached.cppm", "-o",           CacheBMIPath.c_str()};
   std::shared_ptr<CompilerInvocation> Invocation =
       createInvocation(Args, CIOpts);
   ASSERT_TRUE(Invocation);