[clang] [clang][Driver] Add a new member for CLANG_DEFAULT_LINKER to clang::driver::Driver (NFC) (PR #149784)

[Fangrui Song via cfe-commits]

================
@@ -450,6 +454,11 @@ class Driver {
     return ClangExecutable.c_str();
   }
 
+  StringRef getPreferredLinker() const { return PreferredLinker; }
+  void setPreferredLinker(std::string Value) {
----------------
MaskRay wrote:

If #149786 will use `setPreferredLinker`, this `setPreferredLinker` (currently unused) is ok.

https://github.com/llvm/llvm-project/pull/149784