[clang] [clang][lex] Fix non-portability diagnostics with absolute path (PR #74782)

[Steven Wu via cfe-commits]

================
@@ -2466,15 +2466,21 @@ Preprocessor::ImportAction Preprocessor::HandleHeaderIncludeOrImport(
         // The drive letter is optional for absolute paths on Windows, but
         // clang currently cannot process absolute paths in #include lines that
         // don't have a drive.
-        // If the first entry in Components is a directory separator,
-        // then the code at the bottom of this loop that keeps the original
-        // directory separator style copies it. If the second entry is
-        // a directory separator (the C:\ case), then that separator already
-        // got copied when the C: was processed and we want to skip that entry.
-        if (!(Component.size() == 1 && IsSep(Component[0])))
+        if (Component.size() == 1 && IsSep(Component[0])) {
+          // Note: Path always contains at least '<' or '"'.
+          if (Path.size() == 1) {
----------------
cachemeifyoucan wrote:

I feel better if the comment just explains both branches, instead of using `if` to separate comments. I might even consider write a big block to combine with the comments before `if (Component.size() == 1 && IsSep(Component[0]))`. I don't feel the this code is very easy to understand with current comment.

https://github.com/llvm/llvm-project/pull/74782