Driver: Avoid llvm::sys::path::append if resource directory absolute.

After #145996 CLANG_RESOURCE_DIR can be an absolute path so we need to
handle it correctly in the driver.

llvm::sys::path::append does not append absolute paths in the way
that I expected (or consistent with other similar APIs such as C++17
std::filesystem::path::append or Python os.path.join); instead, it
effectively discards the leading / and appends the resulting relative path
(e.g. append(P, "/bar") with P = "/foo" sets P to "/foo/bar").

Many tests start failing if I try to align llvm::sys::path::append with
the other APIs because of callers that expect the existing behavior,
so for now let's add a special case here for absolute resource paths,
and document the behavior in Path.h.

Reviewers: MaskRay

Reviewed By: MaskRay

Pull Request: https://github.com/llvm/llvm-project/pull/146449

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