The `llvm::sys::fs::make_absolute(const Twine &, SmallVectorImpl<char>
&)` functions doesn't perform any FS access - it only modifies the
second parameter via path/string operations. This function should live
in the `llvm::sys::path` namespace for consistency and for making it
easier to spot function calls that perform IO.

For the redirecting VFS, the `'overlay-relative'` option controls
whether external paths should be appended to the overlay directory. This
didn't always work as expected: when the overlay file path itself was
relative, its absolute path was decided by the real FS, not the
underlying VFS, and the resulting external path didn't exist in the
underlying VFS. This PR fixes this issue.

When the root node of the `RedirectingFileSystem` is to be resolved to
the current working directory, we previously consulted the real FS
instead of the provided underlying VFS. This PR fixes that issue.

This PR uses the new-ish `llvm::vfs::FileSystem::visit()` interface to
collect VFS overlay entries from an existing `FileSystem` instance
rather than parsing the VFS YAML file anew. This prevents duplicate
diagnostics as observed by `clang/test/VFS/broken-vfs-module-dep.c`.

This switches to `makeIntrusiveRefCnt<FileSystem>` where creating a new
object, and to passing/returning by `IntrusiveRefCntPtr<FileSystem>`
instead of `FileSystem*` or `FileSystem&`, when dealing with existing
objects.

Part of cleanup #151026.

Identified with misc-include-cleaner.

This patch adds an IsText parameter to the following getBufferForFile,
getBufferForFileImpl. We introduce a new virtual function
openFileForReadBinary which defaults to openFileForRead except in
RealFileSystem which uses the OF_None flag instead of OF_Text.

The default is set to OF_Text instead of OF_None, this change in value
does not affect any other platforms other than z/OS. Setting this
parameter correctly is required to open files on z/OS in the correct
encoding. The IsText parameter is based on the context of where we open
files, for example, in the ASTReader, HeaderMap requires that files
always be opened in binary even though they might be tagged as text.

This reverts commit edf3b277a5f2ebe144827ed47463c22743cac5f9.