Update Paths.cpp to allow llvm 20 build

Author: mcbarton

lib/Interpreter/Paths.cpp

@@ -175,11 +175,29 @@ void CopyIncludePaths(const clang::HeaderSearchOptions& Opts,
       if (withFlags) incpaths.push_back("-isystem");
       break;
 
+      // Option was removed in llvm 20. Git log message below.
+      // git log --grep="index-header"
+      // commit 19b4f17d4c0ae12725050d09f04f85bccc686d8e
+      // Author: Jan Svoboda <[email protected]>
+      // Date:   Thu Oct 31 16:04:35 2024 -0700
+      //
+      //    [clang][lex] Remove `-index-header-map` (#114459)
+      //
+      //    This PR removes the `-index-header-map` functionality from Clang.
+      //    AFAIK this was only used internally at Apple and is now dead code.
+      //    The main motivation behind this change is to enable the removal of
+      //    `HeaderFileInfo::Framework` member and reducing the size of that
+      //    data structure.
+      //
+      //    rdar://84036149
+
+#if CLANG_VERSION_MAJOR < 20
     case frontend::IndexHeaderMap:
       if (!withSystem) continue;
       if (withFlags) incpaths.push_back("-index-header-map");
       if (withFlags) incpaths.push_back(E.IsFramework? "-F" : "-I");
       break;
+#endif
 
     case frontend::CSystem:
       if (!withSystem) continue;