Fix bug in the VFS construction. (#3067)

The constructor accepts a `bool` which we were setting to `true` by
converting from a heap allocated, and thus non-null, pointer. But that
heap allocation was dead and leaked, somewhat obviously. Leak checking
is disabled on our CI at the moment, but this was failing for me locally
with our default build on Linux where it uses ASan.

The `true` value also had no effect because the default argument to the
bool parameter is itself, `true`. :sigh:

It's really sad that this compiled. But it's the same thing as using a
non-null pointer in an `if`. There doesn't seem to be any `clang-tidy`
check for a `new` expression that is implicitly converted to a `bool`
type either. I've filled
https://github.com/llvm/llvm-project/issues/64461 requesting a check or
warning to catch this in the future.

explorer/file_test.cpp

@@ -31,7 +31,7 @@ class ExplorerFileTest : public FileTestBase {
            llvm::raw_pwrite_stream& stdout, llvm::raw_pwrite_stream& stderr)
       -> ErrorOr<bool> override {
     // Create the files in-memory.
-    llvm::vfs::InMemoryFileSystem fs(new llvm::vfs::InMemoryFileSystem());
+    llvm::vfs::InMemoryFileSystem fs;
     for (const auto& test_file : test_files) {
       if (!fs.addFile(test_file.filename, /*ModificationTime=*/0,
                       llvm::MemoryBuffer::getMemBuffer(test_file.content))) {