[clangd] testPath's final result agrees with the passed in Style

[kadircet]

Summary

This was confusing, as testRoot on windows results in C:\\clangd-test
and testPath generated with posix explicitly still contained backslashes.

This patch ensures not only the relative part, but the whole final result
respects passed in Style.

clang-tools-extra/clangd/unittests/ConfigCompileTests.cpp

auto GetFrag = [](llvm::StringRef Directory,
     External.File.emplace(testPath("foo"));
     if (MountPoint)
       External.MountPoint.emplace(*MountPoint);
     Frag.Index.External = std::move(External);
     return Frag;
   };
 
   auto BarPath = testPath("foo/bar.h", llvm::sys::path::Style::posix);
-  BarPath = llvm::sys::path::convert_to_slash(BarPath);
   Parm.Path = BarPath;
   // Non-absolute MountPoint without a directory raises an error.
   Frag = GetFrag("", "foo");
   compileAndApply();
   ASSERT_THAT(
       Diags.Diagnostics,
       ElementsAre(
           AllOf(DiagMessage("MountPoint must be an absolute path, because this "
                             "fragment is not associated with any directory."),
                 DiagKind(llvm::SourceMgr::DK_Error))));
   ASSERT_FALSE(Conf.Index.External);
 
   auto FooPath = testPath("foo/", llvm::sys::path::Style::posix);
-  FooPath = llvm::sys::path::convert_to_slash(FooPath);
   // Ok when relative.
   Frag = GetFrag(testRoot(), "foo/");
   compileAndApply();
   ASSERT_THAT(Diags.Diagnostics, IsEmpty());
   ASSERT_TRUE(Conf.Index.External);
   EXPECT_THAT(Conf.Index.External->MountPoint, FooPath);
 
   // None defaults to ".".
   Frag = GetFrag(FooPath, llvm::None);
   compileAndApply();
   ASSERT_THAT(Diags.Diagnostics, IsEmpty());
   ASSERT_TRUE(Conf.Index.External);
   EXPECT_THAT(Conf.Index.External->MountPoint, FooPath);
 
   // Without a file, external index is empty.
   Parm.Path = "";
   Frag = GetFrag("", FooPath.c_str());
   compileAndApply();
   ASSERT_THAT(Diags.Diagnostics, IsEmpty());
   ASSERT_FALSE(Conf.Index.External);
 
   // File outside MountPoint, no index.
   auto BazPath = testPath("bar/baz.h", llvm::sys::path::Style::posix);
-  BazPath = llvm::sys::path::convert_to_slash(BazPath);
   Parm.Path = BazPath;
   Frag = GetFrag("", FooPath.c_str());
   compileAndApply();
   ASSERT_THAT(Diags.Diagnostics, IsEmpty());
   ASSERT_FALSE(Conf.Index.External);
 
   // File under MountPoint, index should be set.
   BazPath = testPath("foo/baz.h", llvm::sys::path::Style::posix);
-  BazPath = llvm::sys::path::convert_to_slash(BazPath);
   Parm.Path = BazPath;
   Frag = GetFrag("", FooPath.c_str());
   compileAndApply();
   ASSERT_THAT(Diags.Diagnostics, IsEmpty());
   ASSERT_TRUE(Conf.Index.External);
   EXPECT_THAT(Conf.Index.External->MountPoint, FooPath);
 }
 } // namespace
 } // namespace config
 } // namespace clangd
 } // namespace clang

clang-tools-extra/clangd/unittests/TestFS.cpp

 #ifdef _WIN32
   return "C:\\clangd-test";
 #else
   return "/clangd-test";
 #endif
 }
 
 std::string testPath(PathRef File, llvm::sys::path::Style Style) {
-  assert(llvm::sys::path::is_relative(File) && "FileName should be relative");
-
-  llvm::SmallString<32> NativeFile = File;
-  llvm::sys::path::native(NativeFile, Style);
+  assert(llvm::sys::path::is_relative(File, Style));
   llvm::SmallString<32> Path;
-  llvm::sys::path::append(Path, Style, testRoot(), NativeFile);
+  llvm::sys::path::append(Path, testRoot(), File);
+  llvm::sys::path::native(Path, Style);
   return std::string(Path.str());
 }
 
 /// unittest: is a scheme that refers to files relative to testRoot().
 /// URI body is a path relative to testRoot() e.g. unittest:///x.h for
 /// /clangd-test/x.h.
 class TestScheme : public URIScheme {
 public: