[llvm] [llvm][support] Refactor symlink handling and add readlink (PR #184256)

[Fangrui Song via llvm-commits]

================
@@ -445,6 +443,13 @@ std::error_code create_link(const Twine &to, const Twine &from) {
   return std::error_code();
 }
 
+std::error_code create_link(const Twine &to, const Twine &from) {
+  std::error_code EC = create_symlink(to, from);
----------------
MaskRay wrote:

 On Unix, the old code was symlink-only. The new code tries symlink then falls back to hard link. This means create_link on Unix now silently creates a hard link if symlink fails. Is this the desired behavior? I think it's acceptable to change Windows behavior.

https://github.com/llvm/llvm-project/pull/184256