Switch Windows search path options to std::string

A CompilerInvocation object may outlive the input argument list, so we need to copy these argument strings to avoid a use-after-free

Author: z2oh

include/swift/AST/SearchPathOptions.h

@@ -372,10 +372,10 @@ class SearchPathOptions {
                            FrameworkSearchPaths.size() - 1);
   }
 
-  std::optional<StringRef> WinSDKRoot = std::nullopt;
-  std::optional<StringRef> WinSDKVersion = std::nullopt;
-  std::optional<StringRef> VCToolsRoot = std::nullopt;
-  std::optional<StringRef> VCToolsVersion = std::nullopt;
+  std::optional<std::string> WinSDKRoot = std::nullopt;
+  std::optional<std::string> WinSDKVersion = std::nullopt;
+  std::optional<std::string> VCToolsRoot = std::nullopt;
+  std::optional<std::string> VCToolsVersion = std::nullopt;
 
 public:
   StringRef getSDKPath() const { return SDKPath; }