Directly use a vector instead of wrapping it in a stack, like we
do in most places.

When trying to add a file to clang's VFS via `addFile` and a directory
of the same name already exists, we run into a [out-of-bound
access](https://github.com/llvm/llvm-project/blob/145815c180fc82c5a55bf568d01d98d250490a55/llvm/lib/Support/Path.cpp#L244).

The problem is that the file name is [recognised as existing path](
https://github.com/llvm/llvm-project/blob/145815c180fc82c5a55bf568d01d98d250490a55/llvm/lib/Support/VirtualFileSystem.cpp#L896)
and thus continues to process the next part of the path which doesn't
exist.

This patch adds a check if we have reached the last part of the filename
and return false in that case.
This we reject to add a file if a directory of the same name already
exists.

This is in sync with [this
check](https://github.com/llvm/llvm-project/blob/145815c180fc82c5a55bf568d01d98d250490a55/llvm/lib/Support/VirtualFileSystem.cpp#L903)
that rejects adding a path if a file of the same name already exists.

I'm planning to remove StringRef::equals in favor of
StringRef::operator==.

- StringRef::operator==/!= outnumber StringRef::equals by a factor of
  70 under llvm/ in terms of their usage.

- The elimination of StringRef::equals brings StringRef closer to
  std::string_view, which has operator== but not equals.

- S == "foo" is more readable than S.equals("foo"), especially for
  !Long.Expression.equals("str") vs Long.Expression != "str".

Use `VFS.equivalent()`, which follows symlinks, to check if two module
cache paths are equivalent. This prevents a PCH error when building from
a different path that is a symlink of the original.

```
error: PCH was compiled with module cache path '/home/foo/blah/ModuleCache/2IBP1TNT8OR8D', but the path is currently '/data/users/foo/blah/ModuleCache/2IBP1TNT8OR8D'
1 error generated.
```

I'm planning to remove StringRef::equals in favor of
StringRef::operator==.

- StringRef::operator== outnumbers StringRef::equals by a factor of 25
  under llvm/ in terms of their usage.

- The elimination of StringRef::equals brings StringRef closer to
  std::string_view, which has operator== but not equals.

- S == "foo" is more readable than S.equals("foo"), especially for
  !Long.Expression.equals("str") vs Long.Expression != "str".

Allow a `vfs::FileSystem` to provide a more efficient implementation of
`exists()` if they are able to. The existing `FileSystem`
implementations continue to default to using `status()` except that
overlay, proxy, and redirecting filesystems are taught to forward calls
to `exists()` correctly to their wrapped/external filesystem.

Co-authored-by: Ben Langmuir <blangmuir@apple.com>

This is an NFC change split from https://github.com/llvm/llvm-project/pull/68645.

When we lookup in the external filesystem, do not remove . and ..
components from the original path. For .. this is a correctness issue in
the presence of symlinks, while for . it is simply better practice to
preserve the original path to better match the behaviour of other
filesystems. The only modification we need is to apply the working
directory, since it could differ from the external filesystem.

rdar://123655660